Optical fibre with high numerical aperture, method of its production, and use thereof

ABSTRACT

An article comprising an optical fiber, the fiber comprising at least one core surrounded by a first outer cladding region, the first outer cladding region being surrounded by a second outer cladding region, the first outer cladding region in the cross-section comprising a number of first outer cladding features having a lower refractive index than any material surrounding the first outer cladding features, wherein for a plurality of said first outer cladding features, the minimum distance between two nearest neighboring first outer cladding features is smaller than 1.0 μm or smaller than an optical wavelength of light guided through the fiber when in use; a method of its production, and use thereof.

DESCRIPTION

[0001] 1. Background of the Invention

[0002] The present invention relates to electromagnetic waveguides,especially optical fibres having high numerical aperture, such asmultimode optical fibres for high-power delivery and optical fibres,having rare-earth dopants in core and/or cladding region(s) and havingwaveguiding properties designed for high-power amplification and/orlasing, a method of its production, and use thereof.

[0003] 2. The Technical Field

[0004] Over the past few years, cladding pumped fibre lasers andamplifiers have become basic tools in research laboratories. In contrastto conventional optical fibres, which comprise a region of relativelyhigh refractive index the core—surrounded by a region of relatively lowrefractive index—the cladding, cladding pumped (or double-clad) fibresconsist of a high-index core, surrounded by a region of intermediaterefractive index, which in turn is surrounded by a region (typicallypolymer) of low refractive index that does play a role in light guiding.Double clad fibres for instance find use in high-power (cladding pumped)lasers and amplifiers. In such components, pump light from lowbrightness sources, such as diode arrays, is easily coupled into theinner cladding of double clad fibre due to the inner cladding's largecross sectional area and high numerical aperture. As the multimode pumplight crosses the core, it is absorbed by the rare earth dopant, and inorder to increase the overlap of the pump light with the core, the innercladding is often made non-circular. It is important at this point tonote that one of the problems of using circular inner-claddings is thepossible excitation of so-called skew rays, which may be envisioned asoptical rays of a spiral like shape, i.e., rays that do not overlapstrongly with the core region, and, therefore, do not efficiently pumpthe rare-earth materials located in the core. The idea behind suchdouble clad designs is described in recent textbooks (e.g., by Becker,Olsson, and Simpson, “Erbium-Doped Fiber Amplifiers, Fundamentals andTechnology”, Academic Press, 1999, ISBN 0-12-084590-3). However, as itis also described by Becker et al., the inner cladding is often a glasscladding surrounded by a low index polymer second cladding, which allowsthe inner cladding to become a guiding structure. Pump light is launchedfrom the fibre end into the undoped cladding, propagating in a multimodefashion and interacting with the doped core as it travels along thefibre.

[0005] In an evaluation of high power fibre laser, Becker et al.describes that a rectangular shape for the multimode section (the innercladding) is preferred for efficient coupling of radiation to the innersingle-mode core, and matches well the geometric aspect ratio of diodelaser pumps. The output of fibre lasers of this type is constrained tobe single mode by the single mode fibre core, hence the name brightnessconverters (from large area multimode to single mode) is often given tothese devices.

[0006] High-power fibre lasers are often used to replace solid-statelasers, since well-designed fibre lasers offer excellent thermalproperties, reliability, simplicity, and compactness (as described byHodzynski et al. in paper CWA49 of Technical Digest of CLEO'2001, May6-11, 2001, Baltimore, Md., USA). Cladding pumped lasers and amplifiersfind applications not only in telecommunications as high-powererbium-doped fibre amplifiers (EDFAs) and Raman amplifiers, but also inmore traditional applications such as narrow band and single frequencypump sources for optical parametric generators and non-linear frequencyconverters. Also applications such as free space optical communicationlinks, high peak power laser sources are required, and as described byValley et al. in paper CWA51 of the Technical Digest of CLEO'2001, May6-11, 2001, Baltimore, Md., USA, a potential candidate is thehigh-power, cladding pumped, Yb-doped fibre amplifier with apulse-position-modulated seed oscillator. These principles will also bevalid for fibres doped with other rare-earth ions, for operation atother wavelengths. An interesting possibility is described by Söderlundet al. in IEEE Photonics Technology Letters, Vol.13, No.1, January 2001,pp.22-24, in which the amplified spontaneous emission (ASE) is describedin cladding pumped long-wavelength band erbium-doped fibre amplifiers.It is here shown that with cladding pumping, directional effects ofpumping are much reduced by increasing the cladding area. In effect,large cladding area results in more uniform pump power distributionalong the fibre length, preventing build-up of short-wavelength gain andASE power.

[0007] Recently a new type of optical fibre that is characterized by aso-called microstructure has been proposed. Optical fibres of this type(which are referred to by several names—as e.g. micro-structured fibres,photonic crystal fibre, holey fibre, and photonic bandgap fibres) havebeen described in a number of references, such as WO 99/64903, WO99/64904, and Broeng et al (see Pure and Applied Optics, pp.477-482,1999) describing such fibres having claddings defining Photonic Band Gap(PBG) structures, and U.S. Pat. No. 5,802,236, Knight et al. (see J.Opt. Soc. Am. A, Vol. 15, No. 3, pp. 748-752, 1998), Monro et al. (seeOptics Letters, Vol.25 (4), p.206-8, February 2000) defining fibreswhere the light is transmitted using modified Total Internal Reflection(TIR) . This application covers fibres that are mainly guiding by TIR.Micro-structured fibres are known to exhibit waveguiding properties thatare unattainable using conventional fibres.

[0008] In order to increase the amount of pump light that can be coupledinto the fibre, D. J. DiGiovanni and R. S. Windeler has described a newair-clad fibre design in U.S. Pat. No. 5,907,652. DiGiovanni et al.discloses a cladding-pumped optical fibre structure that facilitatesimproved coupling of pump radiation into the fibre. Another aspect ofthe fibres disclosed by DiGiovanni et al. is to optically isolate theinner cladding from the outer structure in order to avoid recoatinginduced changes in optical properties of fibre Bragg gratings written inthe fibre by ultra-violet (UV) light. The fibres according to thedescription of DiGiovanni et al. have increased numerical aperture (NA)resulting from provision of a cladding region having substantially lowereffective refractive index than was found in the prior art. This wasachieved by making the first outer cladding region substantially anair-clad region.

[0009] The application of microstructured fibres—or photonic crystalfibres—in connection with ytterbium-doping has been suggested andreported by W. J. Wadsworth et al. in IEE Electronics Letters, Vol.36,pp.1452-1453, 2000. Furthermore, the issue of high-power levels givingrise to undesired non-linearities or physical damage has been addressedvery recently by W. J. Wadsworth et al. in paper CWCl of the TechnicalDigest of CLEO'2001, May 6-11, 2001, Baltimore, Md., USA. The approachof W. J. Wadsworth et al. is to combine the single-mode and large-modearea properties of photonic crystal fibres with ytterbium codoping.Moreover, it is pointed out by Wadsworth et al. that care must be takenthat any doped regions within the PCF do not themselves form waveguides.To avoid this the core of the presented ytterbium-doped fibre has beenmicrostructured into 425 doped regions with diameters of less that 250nm each—hereby forming an effective index medium with an area fillingfraction of the doped glass of a few percent resulting in an effectivestep, which is insufficient for strong guidance. Wadsworth et al.furthermore mentions the potential of this technology to, scale to evenlarger cores, high output powers and for efficient cladding pumping fromdiode laser arrays using high-numerical-aperture double-cladmicrostructures.

[0010] In a recent publication by Doya, Legrand, and Mortessagne,(Optics Letters, Vol.26, No.12, Jun. 15, 2001, pp.872-874) an optimisedabsorption of pump power is described for a fibre with a D-shaped innercladding. Doya et al. uses a ray trajectory in the transverse section ofthe D-shaped fibre inner cladding to argue a well distributed pumpdistribution (avoiding the previously mentioned skew rays).

[0011] Russell et al. in WO 0142829 describe microstructured fibres foruse as lasers for example as cladding pumped devices. The fibresdescribes by Russell et al. are characterized by a micro-structuredinner cladding having a large (more than 10) number of low-indexfeatures that are arranged in a periodic manner. The inner claddingregion of the fibres describes by Russell et al. are furthercharacterized by a symmetric outer shape—as for example a circular orrectangular shape.

[0012] In order to achieve a high NA, DiGiovanni et al. in U.S. Pat. No.5,907,652 describe that the first outer cladding region (also named theair-clad) should “ . . . to a large extend be empty space, with arelatively small portion (typically <50%, preferably <25%) of the firstouter cladding region being a support structure (the “web”) that fixesthe second outer cladding region relative to the inner cladding region.”As shall be demonstrated under the detailed description of the presentinvention, the present inventors have, however, realised that it is notnecessarily sufficient to have an air-clad region with a large airfilling fraction for the fiber to exhibit a high NA of the innercladding. In fact, the present inventors have realized that it isnecessary to have the “web” designed in a specific manner that relatesto the thickness of the threads of the “web” compared to the opticalwavelength of light guided in the inner cladding in order to achieve ahigh NA.

[0013] It is a disadvantage of the fibres described by DiGiovanni et al.in U.S. Pat. No. 5,907,652 that the “web” has not been optimized for ahigh NA.

[0014] It is a further disadvantage of the fibres described byDiGiovanni et al. in U.S. Pat. No. 5,907,652 that the cross-section ofthe inner-cladding in the shown examples are substantially circular.This may lead to the appearance of skew rays of the pump, and result innonoptimal pumping of the core region.

[0015] It is a further disadvantage of the fibres described byDiGiovanni et al. in U.S. Pat. No. 5,907,652 that the inner cladding isnot microstructured, i.e., control of the effective refractive index ofthe inner cladding has not been described, or moreover the use ofspecific placements of air-holes or voids in the inner cladding has notbeen explored.

[0016] It is a disadvantage of the Yb-codoped fibres reported by W. J.Wadsworth et al. in IEE Electronics Letters, Vol.36, pp.1452-1453, 2000that they do not treat the issue of pump power distribution in multimoderegions such as it is generally used in double clad fibres for highpower applications.

[0017] It is a disadvantage of the D-shaped fibres described by Doya etal. that the advantages of microstructuring have not been used. It is afurther disadvantage that the D-shaped fibre design require significantcomplexity in preform treatment (often involving long time-polishing),which may result in fibre glass defects.

[0018] It is a disadvantage of the fibres presented by Russell et al.that the shape of the inner cladding region is symmetric. It may be afurther disadvantage that the fibres presented by Russell et al. have alarge number of low-index features in the inner cladding region, asthese will act to lower the NA of modes in the inner cladding comparedto inner cladding features having none or a low number of low indexfeatures (less than 10). It may be a further disadvantage of the fibrespresented by Russell et al. that the low-index features of the innercladding region are periodically arranged. The present inventors haverealised that non-periodic arrangement of features in the inner claddingregion may provide a more efficient coupling between cladding modes andmode(s) guided in the fibre core.

DISCLOSURE OF THE INVENTION Object of the Invention

[0019] It is the object of the present invention to provide a new classof optical waveguides, for which improved coupling into cladding pumpedoptical fibres may be obtained through optimal designs ofmicro-structured outer cladding regions that provide high NA for mode(s)of an inner cladding region.

[0020] It is a further object of the present invention to provide a newclass of optical waveguides, in which improved efficiency of claddingpumped optical fibres may be obtained through optimal design ofmicro-structured inner cladding regions. Hereby, a more dynamical designof effective index—and a higher degree of flexibility concerning a givenspatial pump distribution—is possible.

[0021] It is a further object of the present invention to provide newand improved cladding pumped devices in which the mode propagationproperties of the photonic bandgap effect may be combined with amicrostructured inner cladding for high power amplification and lasing

[0022] It is still a further object of the present invention to provideimproved fibre laser and amplifiers, which combines the feasibility ofaccurate spatial mode control of microstructured optical fibres withmultimode pumping properties, and optimal placement of the activemedium, e.g., the rare-earth-doped material.

Solution According to the Invention

[0023] The present inventors have realised that the use of low indexcladding features with a relatively narrow area between neighbouringlow-index features may be required in order to realise cladding pumpedfibre amplifiers and lasers with a high NA of a fibre structure guidinga number of inner, cladding modes. The present inventors have realisedan important relation between width of the above-mentioned areas and theoptical wavelength of inner, cladding modes. The high NA of fibres,which may be obtained in fibres according to the present invention,provides advantages with respect to efficient coupling of light fromlaser sources into the fibres. The high NA may further provideadvantages with respect to efficient transfer of energy from claddingmodes in an inner cladding-to mode(s) of the core.

[0024] According to a first aspect of the invention, there is providedan optical fiber comprising an optical fibre, the fibre comprising atleast one core surrounded by a first outer cladding region, the firstouter cladding region being surrounded by a second outer claddingregion, the first outer cladding region in the cross-section comprisinga number of first outer cladding features having a lower refractiveindex than any material surrounding the first outer cladding features,wherein for a plurality of said first outer cladding features, theminimum distance between two nearest neighbouring first outer claddingfeatures is smaller than 1.0 μm or smaller than an optical wavelength oflight guided through the fibre when in use. Here, the core may besurrounded by an outer cladding, the outer cladding comprising the firstand the second outer cladding regions with the first outer claddingregion arranged between the core and the second outer cladding region.

[0025] For the present invention, the minimum distance between twonearest neighbouring outer cladding features is meant to be the minimumdistance between the outer boundaries of two nearest neighbouringcladding features.

[0026] It should be understood that when looking at a fibre of a givenlength, the cross-sectional dimensions of the fibre may vary along thelength of the fibre. Thus, the present invention covers articles havinga fibre, which in at least one cross-sectional area along the fibrelength is given by one or more of the herein described embodiments.Here, the at least one cross-sectional area may represent an end surfaceof the fibre. It is also within a preferred embodiment that the at leastone cross-sectional area represents a largest cross-sectional area alongthe fibre length.

[0027] According to a preferred embodiment of the invention, the fibremay be dimensioned so that the cross-sectional area of the fibre has avariation along the fibre length, which is not higher than 15% or nothigher than 10%.

[0028] According to an embodiment of the present invention, the firstouter cladding region in the cross section may have an inner diameter orinner cross-sectional dimension being larger than or equal to 15 μm.Here, the inner diameter or inner cross-sectional dimension of the firstouter cladding region may be larger than or equal to 20 μm. Preferably,the inner diameter or inner cross-sectional dimension of the first outercladding region is in the range from 80-125 μm or in the range from125-350 μm.

[0029] The present invention also covers an embodiment in which thefibre comprises a number of cores with a plurality of said number ofcores each being surrounded by a corresponding first outer claddingregion, the cores and the first outer cladding regions being surroundedby the. second outer cladding region, each of the first outer claddingregions in the cross-section comprising a number of first outer claddingfeatures having a lower refractive index than any material surroundingthe first outer cladding features, wherein for a plurality of the firstouter cladding features of each of said first outer cladding regions,the minimum distance between two nearest neighbouring first outercladding features is smaller than 1.0 μm or smaller than an opticalwavelength of light guided through the fibre when in use.

[0030] Thus, according to a second aspect of the present invention,there is provided an article comprising an optical fibre, the fibrecomprising a number of cores with a plurality of said number of coreseach being surrounded by a corresponding first outer cladding region,the cores and the first outer cladding regions being surrounded by asecond outer cladding region, each of the first outer cladding regionsin the cross-section comprising a number of first outer claddingfeatures having a lower refractive index than any material surroundingthe first outer cladding features, wherein for a plurality of the firstouter cladding features of each of said first outer cladding regions,the minimum distance between two nearest neighbouring first outercladding features is smaller than 1.0 μm or smaller than an opticalwavelength of light guided through the fibre when in use.

[0031] In a more specific aspect, the present invention provides anoptical fibre for guiding light of at least one predeterminedwavelength, the optical fiber having a longitudinal direction and across-section perpendicular thereto, the optical fibre comprising:

[0032] (a) at least one core region;

[0033] (b) a cladding region, said cladding region comprising:

[0034] an outer cladding, said outer cladding comprising:

[0035] (i) a first outer cladding region, said first outer claddingregion comprising a first outer cladding background material and aplurality of first outer cladding features, said first outer claddingfeatures having a lower refractive index than said first outer claddingbackground material, and surrounding said at least one core region, and

[0036] (ii) at least one further outer cladding region, each of said atleast one further outer cladding regions comprising a further outercladding background material, and surrounding said first outer claddingregion,

[0037] wherein for a plurality of said first outer cladding features,two nearest neighbouring first outer cladding features have a minimumdistance smaller than the wavelength of said light of at least onepredetermined wavelength whereby an optical fibre having a high NA canbe obtained.

[0038] In another aspect the present invention provides a method ofproducing an optical fibre for guiding light of at least onepredetermined wavelength, the method comprising:

[0039] (a) providing a preform, said preform comprising:

[0040] (i) at least one centre preform element for providing a coreregion of the optical fibre, said center preform element comprising atleast one element selected from the group consisting of rods, tubes, orcombinations thereof;

[0041] (ii) a plurality of inner cladding preform elements for providingan inner cladding region of the optical fibre, said inner claddingpreform elements comprising at least one element selected from the groupconsisting of rods, tubes, or combinations thereof;

[0042] (iii) a plurality of first outer cladding preform elements forproviding a first outer cladding region of the optical fibre, said firstouter cladding preform elements comprising a plurality of elementsselected from the group consisting of rods, tubes, or combinationsthereof;

[0043] (iv) optionally a plurality of further outer cladding preformelements for providing at least one further outer cladding region of theoptical fibre, said further outer cladding preform elements comprising aplurality of elements selected from the group consisting of rods, tubes,or combinations thereof; and

[0044] (v) an overcladding preform element for providing an outerdiameter of the optical fibre, said overcladding preform elementcomprising an element in form of a tube; and

[0045] (b) drawing said preform into a fibre;

[0046] wherein said first outer cladding preform elements are arrangedto provide a minimum distance between two neighbouring first outercladding elements of the optical fibre which is smaller than thewavelength of said light of at least one predetermined wavelength to betransmitted in the optical fibre.

[0047] Preferred embodiments are disclosed in the claims and furtherdiscussed hereinbelow.

[0048] For the embodiments of the present invention having a pluralityof a number of cores each being surrounded by a corresponding firstouter cladding region, it is preferred that each of said number of coresare surrounded by a first outer cladding region. It is also preferredthat at least part of the plurality of cores each being surrounded by acorresponding first outer cladding region are arranged so that the firstouter cladding regions of two neighbouring cores share a number of saidfirst outer cladding features. Here, all of the plurality of cores eachbeing surrounded by a first outer cladding region may be arranged sothat the first outer cladding regions of two neighbouring cores share anumber of said first outer cladding features.

[0049] For the embodiments of the present invention having a pluralityof a number of cores each being surrounded by a corresponding firstouter cladding region, it is preferred that a number or all of the firstouter cladding regions in the cross section have an inner diameter orinner cross-sectional dimension being in the range of 5-100 μm. Here, anumber or all of the first outer cladding regions in the cross sectionmay have an inner diameter or inner cross-sectional dimension being inthe range of 30-60 μm. It is also preferred that the plurality of coreseach being surrounded by a first outer cladding region comprises atleast 20 cores, such as at least 100 cores, such as at least 1000 cores,or such as at least 3000 cores.

[0050] The present invention also covers an embodiment in which thesecond outer cladding region is part of an outer cladding, which furthercomprises third and fourth outer cladding regions with the third outercladding region arranged between the second and the fourth outercladding region, the third outer cladding region in the cross-sectioncomprising a number of third outer cladding features having a lowerrefractive index than any material surrounding the third outer claddingfeatures, wherein for a plurality of said third outer cladding features,the minimum distance between two nearest neighbouring third outercladding features is smaller than 1.0 μm or smaller than an opticalwavelength of light guided through the fibre when in use.

[0051] When for a plurality of said first and/or third outer claddingfeatures, the minimum distance between two nearest neighbouring outercladding features is smaller than 1.0 μm, it is within a preferredembodiment that for a plurality of said first and/or third outercladding features the minimum distance between two nearest neighbouringouter cladding features is smaller than 0.8 μm. Here, the minimumdistance may be smaller than 0.5 μm, such as smaller than 0.4 μm, suchas smaller than 0.3 μm, or such as smaller than 0.2 μm.

[0052] When for a plurality of said first outer cladding features theminimum distance between two nearest neighbouring first outer claddingfeatures is smaller than an optical wavelength of light guided throughthe fibre when in use, it is preferred that for a plurality of saidfirst outer cladding features, the minimum distance between two nearestneighbouring first outer cladding features is smaller than a shortestoptical wavelength of light guided through the fibre. Also, when for aplurality of said third outer cladding features the minimum distancebetween two nearest neighbouring third outer cladding features issmaller than an optical wavelength of light guided through the fibrewhen in use, it is preferred that for a plurality of said third outercladding features, the minimum distance between two nearest neighbouringthird outer cladding features is smaller than a shortest opticalwavelength of light guided through the fibre.

[0053] According to an embodiment of the invention, the core or coresmay have a cross-sectional dimension larger than 25 μm. Here, thecross-sectional dimension may be larger than 50 μm, such as larger than75 μm, or such as larger than 100 μm, thereby causing the fibre to guidelight in multiple modes within the core.

[0054] The present invention further covers an embodiment in which oneor more of the cores is/are surrounded by an inner cladding regionhaving an effective refractive index with a lower value than theeffective refractive index of the core being surrounded, said innercladding regions being part of an inner cladding. Here, each innercladding region may be surrounded by a corresponding first outercladding region. It is preferred that the core or cores is/arecontactingly surrounded by the inner cladding region or regions.

[0055] It is preferred that for one or more first outer cladding regionsthe first outer cladding features occupy a relatively large area of thefirst outer cladding region. Thus, the first outer cladding features mayoccupy 45% or more of the cross-sectional area of the one or more firstouter cladding regions. It is preferred that for all the first outercladding regions the first outer cladding features may occupy 45% ormore of the cross-sectional area of the first outer cladding regions. Itis also within preferred embodiments that, the first outer claddingfeatures may occupy at least 50%, such as at least 60%, or such as atleast 70% of the cross-sectional area of the first outer cladding regionor regions. It is also within an embodiment of the invention that thethird outer cladding features occupy 45% or more of the third outercladding. Here, the third outer cladding features may occupy at least50%, such as at least 60%, or such as at least 70% of thecross-sectional area of the third outer cladding.

[0056] The present inventors have also realised that the application ofmicrostructured cladding regions allow for a much more flexible designand mode-control concerning the multi-mode pump waveguide, and moreoverthat accurate placement of one or a few elongated elements in the innercladding region may provide an optimal distribution of the pump poweralong the fibre amplifier or laser. Thus, according to an embodiment ofthe invention, the inner cladding or one or more of the inner claddingregions in the cross-section may comprise one single inner claddingfeature. Alternatively, the inner cladding or one or more of the innercladding regions in the cross-section may comprise at least two innercladding features, and it is preferred that the inner cladding or one ormore of the inner cladding regions in the cross-section comprise(s) lessthan 10 inner cladding features. The inner cladding features may beplaced in a non-periodic manner.

[0057] According to an embodiment of the present invention, the opticalfibre may comprise at least two cores being surrounded by a common firstouter cladding region. Here, the optical fibre may comprise even morecores, such as at least 7 cores, such as at least 19 cores, or such asat least 37 cores being surrounded by a common first outer claddingregion. The optical fibre may also or alternatively comprise a pluralityof cores in a substantially annular arrangement with the plurality ofcores being surrounded by the common first outer cladding region.Preferably, the plurality of substantially annular arranged cores arearranged in an inner cladding so that a major part of the inner claddingare surrounded by the substantially annular arranged cores, said innercladding having an effective refractive index with a lower value thanthe effective refractive index of each of said substantially annulararranged cores.

[0058] For a fibre having at least two cores surrounded by the commonfirst outer cladding region, the distance between one set of two nearestneighbouring cores may be substantially identical to the distancebetween other sets of two nearest neighbouring cores. It is preferredthat each of said cores being surrounded by a common first outercladding region is surrounded by an inner cladding region having aneffective refractive index with a lower value than the effectiverefractive index of the core being surrounded, said inner claddingregions being part of an inner cladding being surrounded by the commonfirst outer cladding region. Also here, the inner cladding in thecross-section may comprise a number of inner cladding features, such asat least two inner cladding features, such as at least 10 inner claddingfeatures. It is preferred that at least part of the inner claddingfeatures are arranged so that a for an inner cladding region surroundingone of said cores, said inner cladding region comprises several innercladding features. It is also preferred that the inner cladding featuresof an inner cladding region are periodically arranged.

[0059] The present invention also covers an embodiment in which theoptical fibre comprises a core being surrounded by an inner claddinghaving a number of inner cladding features being periodicallydistributed in two dimensions within said inner cladding, said innercladding being surrounded by a first outer cladding region. It ispreferred that the core comprises a core feature having a lowerrefractive index than the refractive index of the core materialsurrounding the core feature. It is also preferred that the periodicallyarrangement of the inner cladding features comprises at least 4 or 5periods in a radial direction from the centre of the inner cladding.

[0060] The first and/or third outer cladding features may be placed in aperiodic or non-periodic manner. However, it is preferred that firstand/or third outer cladding features are placed in a non-circularsymmetric manner. It is also within an embodiment of the invention thatthe first outer cladding region in the cross section has a substantiallyhexagonal like shape.

[0061] The first outer cladding features may be of about equal size, butthe invention also covers embodiments in which first outer claddingfeatures of different size are present.

[0062] It is preferred that the first and/or third outer claddingfeatures and/or the inner cladding features are voids, and it ispreferred that the first and/or third outer cladding features and/or theinner cladding features are filled with vacuum, air, a gas, a liquid ora polymer or a combination thereof. Thus, according to an embodiment ofthe invention, the first and/or third cladding features may be voidsfilled with air.

[0063] For embodiments having an inner cladding with core(s) beingsurrounded by an inner cladding or an inner cladding region, it ispreferred that the core(s) has/have a cross-sectional dimension smallerthan 10 □m. This may adapt the fibre to guide light in the core in asingle mode. It is preferred that the core(s) comprise(s) at least onemember of the group consisting of Ge, Al, P, Sn and B. However, theinvention also covers embodiments in which the core(s) comprise(s) atleast one rare earth, such as Er, Yb, Nd, La, Ho, Dy and/or Tm.

[0064] It is also within embodiments of the present invention that thefibre may have a long period grating or a fibre Bragg grating along atleast a part of the fibre length.

[0065] It should be understood that different materials may be used forproducing the fibre of the invention. Here, the fibre may comprise abackground material being silica, chalcogenide, or another type ofglass. It is also within the invention that the fibre may comprise abackground material being polymer.

[0066] The present inventors have realised how the flexible design ofmicrostructured optical fibres may be used for enhanced couplingefficiency for a pump light source (e.g., a semiconductor laser array),and how such designs may be readily fabricated using a preformfabrication technique that ensures a detailed control of themicro-structured elements.

[0067] One of the fundamental problems to be solved by the invention isto obtain a better transversal and longitudinal pump power distributionin fibre lasers and amplifiers by optimal combinations ofmicrostructured elements in the inner cladding and an improved modecontrol of signal and pump modes in cladding pumped fibres throughmicrostructuring of the outer cladding.

[0068] Thus, for embodiments of the invention comprising an innercladding, the article of the invention may be a cladding pumped fibrelaser or amplifier. Here, the article may be a cladding pumped fibrelaser or amplifier comprising a pump radiation source and a length ofsaid optical fibre. Thus, when the minimum distance between two nearestneighbouring outer cladding features is smaller than a shortest opticalwavelength, the shortest optical wavelength may be determined by thewavelength of said pump radiation source.

[0069] It is also within an embodiment of the invention that the opticalfibre in the cross-section has a non-uniform shape of the inner claddingregion along the fibre length.

[0070] According to an embodiment of the invention, the first and/orthird outer cladding features may be periodically distributed. Hereby,the fibre may exhibit photonic bandgap effects, which may confinecladding modes in the core and/or in the inner cladding region.

[0071] It should be understood that for fibres having an inner cladding,the inner cladding region may act as a reservoir for light generated inthe core. For fibres having an inner cladding, it is also within theinvention that the optical fibre may comprise at least two cores forhigh power broadband amplification. The optical fibres of the presentinvention having an inner cladding may be optically pumpedbi-directionally.

[0072] For fibres having inner cladding features, the fibre corematerial may be the same as the background material of the innercladding region.

[0073] The present invention also covers embodiments wherein, for amajority or all of said first outer cladding features, the minimumdistance between two nearest neighbouring outer cladding features issmaller than 1.0 □m. Here, for a majority or all of said first outercladding features, the minimum distance between two nearest neighbouringouter cladding features may be smaller than 0.8 μm, such as smaller than0.5 μm, such as smaller than 0.4 μm, such as smaller than 0.3 μm, orsuch as smaller than 0.2 μm.

[0074] Furthermore, the present invention also covers embodimentswherein, for a majority or all of said first outer cladding features,the minimum distance between two nearest neighbouring outer claddingfeatures is smaller than an optical wavelength of light guided throughthe fibre when in use. Here, the optical wavelength may be a shortestoptical wavelength of light guided through the fibre when in use.

[0075] Similarly, the present invention also covers embodiments wherein,for a majority or all of said third outer cladding features, the minimumdistance between two nearest neighbouring outer cladding features issmaller than 1.0 μm. Here, for a majority or all of said third outercladding features, the minimum distance between two nearest neighbouringouter cladding features may be smaller than 0.8 μm, such as smaller than0.5 μm, such as smaller than 0.4 μm, such as smaller than 0.3 μm, orsuch as smaller than 0.2 μm. Furthermore, the present invention alsocovers embodiments wherein, for a majority or all of said third outercladding features, the minimum distance between two nearest neighbouringouter cladding features is smaller than an optical wavelength of lightguided through the fibre when in use. Here, the optical wavelength maybe a shortest optical wavelength of light guided through the fibre whenin use.

[0076] It should be understood that it is within a preferred embodimentof the invention that the outer cladding has an effective refractive nowith a value lower than the effective refractive index of the core orany of the cores. It is also preferred that the outer cladding has aneffective refractive index no, with a value lower that the effectiverefractive index of the inner cladding or inner cladding regions. Here,the effective refractive index of the inner cladding or inner claddingregions should be lower than the effective refractive index of the coreor any of the cores.

[0077] According to an embodiment of the invention the second outercladding region may be made of a homogeneous material.

[0078] For the first outer cladding features it is within an embodimentof the invention that the largest cross-sectional dimension are equal toor below 10 μm. Here, the largest cross-sectional dimension of the firstouter cladding features may be equal to or below 3 μm.

[0079] For fibres according to the present invention having an innercladding, it is within an embodiment of the invention that the innercladding comprises a background material and a plurality of features,said features having a refractive index being higher and/or lower thanthe refractive index of the background material of the inner cladding.Here, the inner features may be substantially periodically arrangedwithin two dimensions, or the inner features may be radial periodicallyarranged. It is preferred that the inner cladding features have a lowerindex than the refractive index of the background material, and theinner cladding features may be voids. The inner cladding features may befilled with vacuum, air, a gas, a liquid or a polymer or a combinationthereof. When the inner cladding features have a lower index than therefractive index of the inner cladding background material, the innercladding features may have a cross-sectional diameter or dimension inthe range of 0.3-0.6 times or 0.3-0.5 times a centre-to-centre spacingbetween neighbouring inner cladding features. Here, the centre-to-centrespacing between neighbouring inner cladding features may be an averagecentre-to-centre spacing. The centre-to-centre spacing may be largerthan 5 μm, such as in the range of 8-12 μm, or such as about 10 m. It isalso within one or more embodiments of the invention that the corecomprises one or more dopants for raising or lowering the refractiveindex above the refractive index of the background material of the innercladding. The outer diameter of the inner cladding may be within therange from 60-400 μm, or within the range from 200-400 μm.

[0080] The present invention furthermore covers embodiments wherein theoptical fibre has a length with a first end and a second end, andwherein the cross-sectional area of the first outer cladding features inthe first end are larger than any cross-sectional area of first outercladding features in the second end. Here, the second end may compriseno first outer cladding features, or the first outer cladding featuresmay be fully collapsed in the second end.

[0081] Similarly, the present invention covers embodiments wherein theoptical fibre has a length with a first end and a second end, andwherein the cross-sectional area of the third outer cladding features inthe first end are larger than any cross-sectional area of third outercladding features in the second end. Also here, the second end maycomprise no third outer cladding features, or the third outer claddingfeatures may be fully collapsed in the second end.

[0082] It should be understood that according to the present invention,the first outer cladding features may be elongate features extending ina fibre axial direction. Similarly, the third outer cladding featuresmay be elongate features extending in a fibre axial direction, and alsothe inner cladding features may be elongate features extending in afibre axial direction.

[0083] For fibres having an inner cladding it is within an embodiment ofthe present invention that the background material surrounding the firstouter cladding features or the bridging material fulfilling the areabetween neighbouring first outer cladding features has a lowerrefractive index than the refractive index of the background material ofthe inner cladding.

[0084] It is also within the present invention that the articleaccording to one or more embodiments of the invention is an endoscope.

[0085] It is to be understood that both the foregoing generaldescription and the following detailed description are merely exemplaryof the invention, and are intended to provide an overview or frameworkfor understanding the nature and character of the invention as it isclaimed. The accompanying figures are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of the invention. The invention is not limited to the describedexamples. The figures illustrate various features and embodiments of theinvention, and together with the description serve to explain theprinciples and operation of the invention. Where indicated, the figuresare used to describe prior art.

Definition of Expressions and Terms

[0086] The term “article comprising an optical fiber” is intended to beinterpreted broadly. For example it includes the optical fiber itself,e.g. an optical fiber coated with a fiber coating; an optical fiberproduct comprising the optical fiber in e.g. cabling; an optical fiberproduct comprising the optical fiber as an optical component; or anoptical communication system or parts thereof comprising the opticalfibre.

[0087] For micro-structures, a directly measurable quantity is theso-called “filling fraction” that is the volume of disposed features ina micro-structure relative to the total volume of a micro-structure. Forfibres that are invariant in the axial fibre direction, the fillingfraction may be determined from direct inspection of the fibrecross-section.

[0088] In this application we distinguish between “refractive index”,“geometrical index” and “effective index”. The refractive index is theconventional refractive index of a homogeneous material. The geometricalindex of a structure is the geometrically weighted refractive index ofthe structure. As an example, a structure consisting of 40% air(refractive index=1.0 ) and 60% silica (refractive index≈1.45) has ageometrical index of 0.4×1.0+0.6×1.45=1.27. The procedure of determiningthe effective refractive index, which for short is referred to as theeffective index, of a given microstructure at a given wavelength iswell-known to those skilled in the art (see e.g., Joannopoulos et al.,“Photonic Crystals”, Princeton University Press, 1995 or Broeng et al.,Optical Fiber Technology, Vol. 5, pp.305-330, 1999).

[0089] Usually, a numerical method capable of solving Maxwell's equationon full vectorial form is required for accurate determination of theeffective indices of microstructures. The present invention makes use ofemploying such a method that has been well-documented in the literature(see previous Joannopoulos-reference) . In the long-wavelength regime,the effective index is roughly identical to the weighted average of therefractive indices of the constituents of the material, that is, theeffective index is close to the geometrical index in this wavelengthregime. Naturally, for a homogeneous medium, the effective refractiveindex is identical to the refractive index.

BRIEF DESCRIPTION OF THE DRAWINGS

[0090] The functionality and additional features of the invention willbecome clearer upon consideration of the different embodiments now to bedescribed in detail in connection with the accompanying drawings. In thefigures:

[0091]FIG. 1 shows a double-clad fibre according to prior art, in whichthe inner cladding is circular.

[0092]FIG. 2 illustrates a double-clad fibre according to prior art, inwhich the inner cladding is elongated in one transverse direction.

[0093]FIG. 3 illustrates a double-clad fibre according to prior art, inwhich the inner cladding is non-circular, having a leafed cross-section.

[0094]FIG. 4 shows an example of a double-clad fibre according to priorart, in which the first outer cladding is substantially an air-cladregion.

[0095]FIG. 5 illustrates the silica bridging regions that exists aroundthe interface of the inner and outer cladding. A minimum width of abridging region is defined.

[0096]FIG. 6 illustrates the numerical aperture of air-clad fibreshaving a fixed air filling fraction of about 45%, but various widths ofthe bridging region.

[0097]FIG. 7 illustrates the numerical aperture of air-clad fibreshaving a fixed air filling fraction of about 58%, but various widths ofthe bridging region.

[0098]FIG. 8. compares the numerical aperture of air-clad fibres havingdifferent air filling fractions for similar bridging widths.

[0099]FIG. 9 shows a photonic crystal fibre cross section according toprior art in which the effective refractive index of the inner claddingis determined by a large number of periodically arranged, smallerair-holes and the effective refractive index of the outer cladding isdetermined by air-holes of a different cross section compared to thoseof the inner cladding.

[0100]FIG. 10 illustrates an example of a photonic crystal fibreaccording to the invention, in which a small number of non-periodicallyarranged air-holes are used as extra elements in the inner cladding toassist cladding pumping.

[0101]FIG. 11 illustrates an example of a cross-section of a photoniccrystal fibre according to the invention, in which the air holes formingthe second cladding are positioned non-circularly.

[0102]FIG. 12 illustrates an example of a cross-section of a photoniccrystal fibre according to the invention, in which the air holes formingthe second cladding are of various sizes.

[0103]FIG. 13 shows cross-section examples of photonic crystal fibresaccording to the invention, in which the air holes forming the innercladding are positioned non-circularly.

[0104]FIG. 14 illustrates an example of a cross-section of a photoniccrystal fibre preform according to the invention, in which the inner andouter cladding are formed using capillaries of different air-fillingfractions and the inner cladding comprises solid rods.

[0105]FIG. 15 illustrates an example of a cross-section of a photoniccrystal fibre preform according to the invention, in which differentlyshaped capillaries are used to form an asymmetrical structure optimisedfor high coupling efficiency to laser diodes strips.

[0106]FIG. 16 shows an example of a photonic crystal fibre transitionelement designed for high coupling efficiency between a non-circularsymmetric mode and a near circular symmetric mode.

[0107]FIG. 17 shows another example of a photonic crystal fibreaccording to the present invention. The fibre is of a general typehaving a high effective index contrast between adjacent cladding layers.

[0108]FIG. 18 shows another example of a photonic crystal fibreaccording to the present invention. The fibre has a large homogeneouscore region surrounded by an air-clad region. The fibre is highlymultimode and has a high NA.

[0109]FIG. 19 shows two examples of photonic crystal fibres according tothe present invention. The fibres have a high NA in one end of theirlength (for example an input end where light is coupled into the fibre)and a low NA in the other end. The high and low NA is obtained bycontrolling the air filling fraction in the air-clad region along thelength of the fibres—or alternatively by collapsing the air holes at theend-facet in one end of the fibres. FIG. 19a shows a multimode fibre,and FIG. 19b shows a fibre with a doped core to provide single modeoperation at a given wavelength in the doped core.

[0110]FIG. 20 shows another example of a photonic crystal fibreaccording to the present invention. The fibre has a large core regionwhere light may be guided in a single mode. The fibre further has amicrostructure in the inner cladding acting to support only single modein the core and an air-clad region acting to provide a high NA for modesin the inner cladding.

[0111]FIG. 21 shows an example of a real photonic crystal fibreaccording to the present invention. The fibre has an air-clad regionwith narrow bridging regions that provide a high NA of the fibre.

[0112]FIG. 22 illustrates schematically the parameter, T, which is usedto characterise the thickness of the air-clad layer.

[0113]FIG. 23 shows another example of a real photonic crystal fibreaccording to the present invention. The fibre has an air-clad regionwith narrow bridging regions that provide a high NA of the fibre. Thefibre in FIG. 22a has a larger thickness of the air-clad region than thefibre shown in FIG. 21, but the NA of the two fibres is practicallyidentical. FIG. 22b shows the complete cross-section of the fibre.

[0114]FIG. 24 shows schematically an example of a fibre according to theresent invention for use in endoscopes. The fibre comprises a largenumber (typically more than 100) multi-core elements that are separatedfrom each other using air-clad layerswith thin bridging regions.

[0115]FIG. 25 shows a multi-core fibre according to the presentinvention. The fibre may be used as a high-brightness laser.

[0116]FIG. 26 shows another example of a multi-core fibre according tothe present invention.

[0117]FIG. 27 shows schematically an optical fibre according to thepresent invention, wherein periodically distributed features in theinner cladding provides confinement of signal light using PBG effect.

[0118]FIG. 28 shows schematic the operation of a fibre amplifier or afibre laser, where improved efficiency is obtained through the use ofPBG effect.

[0119]FIG. 29 shows experimental and simulated results of NA for twofibres with different bridging widths as a function of wavelength.

[0120]FIG. 30 shows experimental and simulated results of NA as afunction of wavelength divided by the bridging width.

[0121]FIG. 31 shows schematically the cross-section of an optical fibreaccording to the present invention having a lower refractive index inthe bridging regions compared to the background material of the innercladding region.

[0122]FIG. 32 shows schematically the refractive index profile along onedirection in the cross-section of a fibre according to the presentinvention having a lower refractive index in the bridging regionscompared to the background material of the inner cladding region.

[0123]FIG. 33 shows simulated results of NA as a function of wavelengthdivided by the bridging width for fibres with different refractive indexcontrasts between the bridging regions and the background material ofthe inner cladding region.

DETAILED DESCRIPTION

[0124] This description of the present invention is based on examples.The invention is in no way limited to the presented examples that merelyact to illustrate the concepts and design ideas that underlie theinvention.

[0125]FIG. 1 shows an example of a typical double clad fibre known inthe prior art. This type of fibre is widely used for cladding pumpedfibre amplifiers and lasers. The fibre is characterised by a core region10 and two cladding regions; an inner cladding region 11 and an outercladding region 12. Typically, the refractive index of the core regionis higher than that of the inner cladding region, whereby the core mayacts as a core in a conventional single mode optical fibre, and theinner cladding region has a higher refractive index than that of theouter cladding region, whereby a number of cladding modes may be guidedin the inner cladding. The principle of the fibre as a cladding pumpedamplifier or laser device is merely that pumping of an active materialin the fibre core is facilitated using the cladding modes as means fortransferring pump light from a pump laser to the core. High power lasersare typically multi-mode and may more efficiently be coupled to claddingmodes of the double clad fibre than directly to a mode in the core. Bytransfer of optical energy from the cladding modes to the core modealong the fibre length, an overall more efficient pumping may beachieved compared to direct coupling of pump laser light into the fibrecore. In the section regarding background of the invention a number ofreferences to this type of fibre device may be found (see also U.S. Pat.No. 5,937,134).

[0126] To improve the transfer of energy from cladding modes to a coremode, a non-circular shape of the inner cladding region is oftenemployed. FIG. 2 shows an example of a prior art double clad fibrehaving a nearly rectangular shaped inner cladding region 21 thatsurrounds the core region 20. The outer cladding region 22 ischaracterized by a lower refractive index than the inner claddingregion, as for the fibre in FIG. 1. The advantages of using non-circularsymmetric shape of the inner cladding have been described in thebackground of the invention. Additionally, the shape of the innercladding region and the incorporation of stress-applying features in theinner cladding feature may be utilized to achieve birefringence indouble clad fibres, such as for example for polarization maintainingapplications (see U.S. Pat. No. 5,949,941 for examples of such fibres).FIG. 3 shows another example of a prior art fibre having non-circularshape of the inner cladding 31.

[0127] When optimising cladding pumped fibres and fibre devices, a firstimportant issue to address is the realization of a large index contrastbetween the inner cladding region and its outwards surroundings. This isimportant in order to have a high numerical aperture, NA, of the innercladding such that efficient coupling from a pump laser to the claddingmodes may be achieved. Typically, the numerical aperture of pump lasers,such as for example multi-mode solid state lasers, is significant largerthan 0.2. Hence, it is desired to realise fibre designs having an innercladding region with a NA larger than 0.2 at the pump wavelength.Secondly, it is important to match the field distribution, both inspatial size and shape, of the cladding modes to the pump fibre modes.Thirdly, it is important that the fibre has an efficient transfer ofenergy from cladding modes to the core.

[0128] An example of a prior art double clad fibre having a potentiallylarge NA of the inner cladding 41 is illustrated in FIG. 4. This fibreis a so-called air-clad fibre that is characterized by the outercladding being divided in two regions; a first outer cladding regioncomprising a number of low-index features (typically air holes) 42 and asecond outer cladding region 43 surrounding the first outer claddingregion and mainly acting as an overcladding layer that providesmechanical support and stability of the fibre. In U.S. Pat. No.5,907,652, DiGiovanni et al. describe this type of air-clad fibre.DiGiovanni et al. point out that it is an advantage to use air holes inorder to achieve a low effective refractive index of the first outercladding region. DiGiovanni et al. state that the effective refractiveindex is determined by the air-filling fraction and in order to improvethe fibre the air filling fraction should be as large as possible. Inpreferred embodiments, the fibre of DiGiovanni et al. therefore havemore than 50% air filling fraction, and further preferred embodimentshave more than 75% of air in the air clad layer (referred to as a“web”). Generally, it is well understood that the effective index ofmicrostructures may be lower by increasing the air filling fraction (orfraction of low index features), it therefore appears a straight-forwardimprovement to increase the air filling fraction in the fibres presentedby DiGiovanni et al. (equivalent to increasing the air filling fraction,DiGiovanni et al. state that the amount of high index material(typically silica is the background material of the “web”) in the airclad region should be reduced—preferably below 50% or further below25%).

[0129] By using a detailed theoretical analysis of the NA of theair-clad fibres, it turns out that the statement presented by DiGiovanniet al. regarding improvements of air-clad fibre is too simple andfocuses only on the air-filling fraction. In fact, it turns out that alarge air-filling fraction in certain cases is no advantage to claddingpumped fibres. On the other hand, it surprisingly turns out that thewidth of high index material in the first outer cladding region is animportant parameter for optimising air-clad fibre and which parametercan be tuned with respect to the optical wavelength of the light guidedthrough the optical fibre. Looking at the cross-section of an air-cladfibre, the parameter in question is the thickness of the threads in the“web” which threads are comprised in the air-clad layer. Morespecifically, the parameter, labelled b, is “the smallest width of highindex material in the first outer cladding region” as illustrated inFIG. 5 for two different air-clad fibres. In FIGS. 5a and 5 b, bothfibres have a core region 52, an inner cladding region 53, a first outercladding region that comprises low-index features 50 and 51,respectively, and a second outer cladding region 54. The parameter b isindicated for both fibres. The parameter, b, may also be seen as thedistance between two features meaning the minimum distance between edgesof two neighbouring low-index features. In the case of periodicallydistributed low-index features in the first outer cladding, such asindicated in FIG. 5a, it should be clear that b is independent of whichtwo low-index features are used to define b. On the other hand, in thecase of non-periodic low-index features—or air-clad structure with somestructural fluctuations that often occur due to fabrication—b will notbe uniform throughout the first outer cladding—see FIG. 5b. In thislatter case, the invention will relate to typical representative valuesof b, a plurality of the possible b's, a majority of b's or all b's.

[0130] A theoretical tool for analysing air-clad fibres is a full-vectornumerical computer program that has been extensively tested and is welldescribed in literature (see Johnson et al., Optics Express 8, no. 3,173-190 (2001)).

[0131] In order to understand the findings of the present inventors,FIG. 6 shows the NA relating to the cladding modes for an air-clad fibrewith a design as schematically shown in FIG. 5a. The fibre has amoderate air-filling fraction in the first outer cladding region ofabout 45% (hence below that of the stated preferred embodiments of theair-clad fibres by DiGiovanni et al. in U.S. Pat. No. 5,907,652).Looking at FIG. 6, it is seen that by tuning the parameter b, to 0.6, μmor lower, it is possible to achieve NA of more than 0.2 over awavelength range X larger than from about 0.8 μm to 2.0 μm. Typicallypreferred pump wavelengths for Erbium doped fibre amplifiers and laserare about 0.98 μm and about 1.48 μm. For fibre amplifiers and laserswith other rare-earth dopants, such as for example Yb, preferred pumpingwavelengths are about 1.06 μm. From FIG. 6, it is also found that largerdimensions of b than 0.6 μm are not an advantage when the air fillingfraction is limited to about 45%. Hence, the air-filling fraction aloneis not a sufficient parameter to adjust when optimising an air-cladfibre to have a large NA.

[0132] Looking at a similar air-clad fibre as in FIG. 6, but having alarger air-filling fraction of the first outer cladding, namely about58%, it is again found that the parameter b plays an important role whenoptimising the NA—see FIG. 7. For the important wavelength range ofabout 0.98 μm to 2.0 μm, it is found that to have an NA of more than0.2, the b parameter has to be smaller than 0.8 μm. As previouslystated, a large air-filling fraction may not necessarily provide a highNA. This is seen from FIG. 7 where the NA is lower than 0.2 for b largerthan 0.8 times the optical wavelength. On the other hand, the sameair-filling fraction may provide a very high NA—of more than 0.5—if b issmaller than 0.2 times the optical wavelength (we refer only to thefree-space optical wavelength in the present invention). Also it is seenthat NA of higher than 0.3 may be achieved for b smaller than 0.4 timesthe optical wave-length.

[0133] While FIG. 6 and FIG. 7 address only two different air fillingfractions, it turns out that to have a NA of about 0.2 or larger, it isrequired that b is not larger than the optical wavelength of lightguided through the fibre. Hence, for pumping about 0.98 μm, it isrequired that b is smaller than 1.0 μm.

[0134] With the teachings of DiGiovanni et al, it may appear surprisingthat the parameter b plays such an important role regarding the NA ofair clad fibres. Following the teachings of DiGiovanni et al., it may beeven more surprising to see that the same NA may actually be achieve fortwo fibres with different air filling fractions, but similar value ofthe b parameter. This is, however, what the present inventors havefound—as may be seen from FIG. 8. The figure shows the NA of a fibrewith an air filling fraction of about 58% and b of 0.2 μm (top curve).Further, the figure compares the NA of two fibres with similar b valueof 0.3 μm, but different air filling fractions of about 45% and of about58% (curves labelled d/Λ=0.7 and d/Λ=0.8, respectively). From these twocurves it is found that despite the different airfilling fraction, theNA of the two fibres is almost identical over the broad wavelength rangefrom about 0.8 μm to 2.0 μm. This result further shows the importance ofthe b parameter regarding the NA of the air clad fibres and how focusingalone on the air filling fraction for the optimisation of the fibres isa too simple approach. To demonstrate that the NA is not only by chancecoinciding for the fibres having b=0.3 μm, FIG. 8 furthermore shows thatthis is also the case for fibres with b=0.4 μm.

[0135] As seen from FIG. 6 to 8, the NA of the fibres is decreasing atshorter wavelengths. This decrease is related to the larger bridgingwidth relative to the optical wavelength. It turns out that it is afurther advantage that the bridging material has a lower refractiveindex than the background material of the inner cladding region. For agiven desired NA of a fibre, this allows an increase of bridging width,or alternatively for a given desired bridging width, it allows a higherNA to be achieved. These aspects allowing increased bridging width for agiven NA, may prove advantageous for issues relating to mechanicalrobustness and handling of the fibres, such as for example cleaving andsplicing—as shall be discussed at a later stage of the presentapplication. Therefore, in preferred embodiments, the air-clad layercomprises low-index features placed in a background material having arefractive index being at least 0.5% lower than a refractive index ofthe background refractive index of the inner cladding region.Preferably, the index difference is larger, such as larger than 1%, or2%, or larger. Such differences may well be achieved using silica-dopingtechniques by for example using F-doped silica glass for the backgroundmaterial of the air-clad layer and/or using Ge-doped glass for thebackground material of the inner cladding region. Using other types ofglasses—such as non-silica glasses—even larger index differences may beachieved. Therefore, in further preferred embodiments, theafore-mentioned index difference is larger than 5%, or larger than 10%.For index difference of about 10% or less, typically the increase inbridging width that can be obtained is relatively small for fibreshaving NA of about 0.5 or larger. Hence, in preferred embodiments, thebridging widths are in the range of about 200 nm to 400 nm for fibreswith NA of about 0.5 or larger.

[0136] Having focused on the air-clad region, it is naturally alsoimportant to notice that a microstructuring of the inner cladding regionmay influence the NA. In this respect, a large number ofmicro-structured, low-index features in the inner cladding region maydegrade the NA of the cladding modes. Hence, fibre designs having a(large) number of large inner cladding features, as presented in theprior art, may not be advantageous (see previously mentioned Russell etal. reference under the background of the invention section). An exampleof such a fibre is shown in FIG. 9. An advantage of micro-strucuring ofthe inner cladding region is, however, that the microstructuring may beused to tailor the mode field distribution in the inner cladding—or toscramble the cladding modes—to provide an improved overlap with a coremode. It turns out that to optimise air-clad fibre for cladding pumpedapplications, a relatively low number of inner cladding features shouldbe employed—in order not to degrade severely the NA of the fibre. It mayfurther be preferred to place these features in non-periodic manners. Anexample of a preferred embodiment of such a fibre according to thepresent invention is illustrated in FIG. 10. The fibre has a rare earthdoped core 100 and an inner cladding region comprising a backgroundmaterial 102 and a low number—here 5—low-index features 101 that arenon-periodically positioned. The low-index features in the innercladding region may be of different size. Surrounding the inner claddingregion is an outer cladding region 103. This outer cladding region maybe a single outer region or may comprise a first, a second or more outercladding regions.

[0137] Another embodiment is shown in FIG. 11. The shape of the innercladding region 111 may also be non-circular using a non-circulararrangement of low-index features 112 in a first outer cladding region.The core 110 of this fibre is also shown. Another example of a preferredembodiment of the optical fibre according to the present invention isindicated in FIG. 12, here the core 120 is surrounded by an innercladding region 121 that has a non-circular outer shape that has beenachieved by using different-sized, low-index features 122, 123 in afirst outer cladding region. FIG. 13 shows other examples of preferredembodiments of the optical fibres according to the present invention,where the core 130 is surrounded by an inner cladding region thatcomprises a background material 131 and low number of features 132 thatare positioned close to the core 130 such that they may affect thewaveguiding of the core mode. The fibre is further characterized by anouter air-clad region 133. Other variations of this type of fibre designis also shown in FIG. 13, including examples of preferred embodiments ofthe optical fibres according to the present invention where the core ismade of the same material as the background material of the innercladding, and the inner cladding features ensure the guidance of asingle mode in the core. As a further note to FIG. 13, it should bementioned that the presence of low-index features around the core mayimpair the coupling between inner cladding mode and the core, thereforeit may be an advantage that the hole pattern is not circular symmetricin order to broaden a few “channels” from the outer part of the innercladding to the core. This may further be used to create a strongbirefringence in the fibres for polarization maintaining applications.It is further important to notice that the coupling between claddingmodes and core mode(s) will be lower for long wavelengths compared toshort wavelengths. Since a short pump wavelength compared to the signalwavelength is often used for cladding pumped fibre amplifiers and laser,the presence of low-index features close to the core may be advantageousfor controlling the waveguidance of the (long wavelength) signal light,while the presence may be of little or none effect for transfer ofenergy from the (short wavelength) cladding modes to the core region—inother words, the presence do allow an efficient coupling from claddingmodes to core mode.

[0138] In order to produce optical fibres according to the presentinvention, a technique well known for fabrication of microstructuredfibres may be employed, see for example U.S. Pat. No. 5,907,652, or anyof the afore-mentioned references. This method has been adapted toproduce embodiments of the optical fibers according to the presentinvention. The method is based on stacking of capillary tubes and rodsto form a preform and drawing this into fibre using a conventionaldrawing tower. The present invention also covers designs of preforms,and an example of a preform according to the present invention isillustrated in FIG. 14. The preform comprises a rare earth doped centreelement 144 that will act as the core in the final fibre. More rods,tubes or a combination of these may also form the core region.Surrounding the core is a number of solid rods (typically undopedsilica) 142, and a few tubes 143 placed in a non-periodic manner. Theserods and tubes form the inner cladding region of the final fibre. Thepreform further comprises a region of capillary tubes 141 having alarger air-filling fraction compared to the tubes in the inner claddingregion. Thereby a smaller thickness, 0.5b_(preform), of the tube wall isobtained. As the preform is reduced in size during drawing of the fiber,the smaller thickness of the tube walls for the tubes in the air cladregion provides the b-parameter of the final fibre. These tubes willform the first outer cladding region—or the air clad region—of the finalfibre. Finally, the fibre preform comprises a large overcladding tube140 that will act as a second outer cladding region providing a desiredouter diameter of the final fibre as well as mechanical robustness ofthe fibre.

[0139] The present invention also covers preforms with designs as shownschematically in FIG. 15. This type of preforms—and the optical fibresthat may be drawn from them—is of importance for coupling from laserdiode strips to cladding pumped optical fibres. Regarding high powerlaser diodes, the preferred pump sources for high power optical fibreamplifiers and lasers typically emit light from a section of dimensionsfrom about 1 μm to several hundred μm. Optical fibres that match variousgeometries may easily be made by stacking capillaries 150 to form anair-clad region and rods 151, 152 to form an inner cladding regionsurrounding a core 153. Moreover, the high NA provided by the air-glassrefractive index contrast makes it possible to guide all the pump lightfrom the diode using direct butting, without having to resort to a lensto collimate the very diverging fast-axis. Preforms build fromnon-circular tube/rods are also possible, see examples of such elements154, 155.

[0140] The present invention also covers optical fibres where the outershape of the fibre changes along the optical fibre length. For example,as shown schematically in FIG. 16, the optical fibre 160 may be“circularised” in one end by heating up the fibre over part of itslength. The fibre may in such a way be tailored for a specific modeprofile in one end, such as a rectangular shape using rectangular placedfeatures 161, and a circular shape using circular placed features 162 inthe other end.

[0141] Hereby, the optical fibre may be adapted to a specific laser atthe input end and the optical fibre may be adapted to exhibit a moresymmetric output beam. The fibre may also be stretched in the samefabrication process to reduce the output spot-size.

[0142] As illustrated in FIG. 16, the fibre may also contain a corematerial that is identical to the inner cladding material—hence the coreand inner cladding acting as one large multimode core.

[0143] Other embodiments according to the present invention maygenerallyinclude microstructured optical fibres having large effective refractiveindex contrast between various regions in the cross-section, such as forexample concentric, annular microstructured regions. Therefore, inanother aspect, the present invention covers microstructured opticalfibres having at least two microstructured cladding regions that areseparated by at least one homogeneous region. An example of such a fibreis illustrated in FIG. 17, where the two microstructured regions areformed from low-index features 170 and 171. The microstructured regionseach have b parameter smaller than 1.0 μm, such as smaller than 0.5 μmor smaller than 0.3 μm to provide a large effective index contrastbetween adjacent regions. The homogeneous region separating the twomicrostructured regions is indicated with numeral 172.

[0144] The present invention discloses how to realize high numericalaperture in optical fibres with an air-clad layer. High numericalaperture is not only of interest for cladding pumped optical fibrelasers and amplifiers, and the present invention also includes otheroptical fibre applications, where narrow bridges are used to obtain highnumerical aperture. As an example, the present invention also coversmultimode optical fibres, where the core region does not containrare-earth elements, and even multimode optical fibres where the coredoes not contain any doping elements at all. Compared to the previouslydiscussed examples of preferred embodiments of the present invention, amulti-mode optical fibre according to the present invention may have acore that is identical to the inner cladding. Hence, the presentinvention covers optical fibres with an air-clad region having narrowbridging regions, where the air-clad region surrounds a large core ofhomogeneous material—for example pure silica. Naturally, the core mayalso comprise doped silica glass to realize a special refractive indexprofile, such as for example a parabolic refractive index profile in thecore. An example of a multimode optical fibre with a high numericalaperture is schematically illustrated in FIG. 18. The optical fibre hasa large core region 180 with a diameter of more than 10 μm, such aslarger than 25 μm. Surrounding the core region 180 is an air-clad layercomprising air holes 181 and, outside the air-clad layer is ahomogeneous cladding layer 182 providing mechanical robustness to thefibres. It is required that the bridging width between the air holes 181is narrow—smaller than 1.0 μm, and preferably smaller than 0.5 μm—inorder for the optical fibre to have a high numerical aperture—largerthan 0.5, and preferably larger than 0.7.

[0145] A multimode optical fibre as the one disclosed above, may forexample be employed for high power delivery. As an example, a multimodefibre may be used as delivery medium for pump sources to various typesof lasers and laser components. Typical for such a (passive) deliveryoptical fibre, it is advantageous to have a high NA at both opticalfibre ends, or only at a single of the optical fibre ends, such as ahigh NA at an input-end and a lower NA at an output-end, or vice versa.For example, as illustrated in FIG. 19a, at an input-end 190 theair-clad layer 191 may be designed with narrow bridging regions forproviding an NA of more than 0.5, and at an output-end 192 a lower NAmay be desired (for example below 0.3)—for mode matching to a standardfibre or a specific laser component—and the holes in the air-clad layermay be partly or fully collapsed. This may either be performed over thefull fibre length, or parts thereof, or directly at the end-facet of theoutput end of the fibre. Such a difference in NA between two ends of anoptical fibre, may also be relevant for active fibres comprising a dopedcore region 195, 197, see FIG. 19b. For example for a cladding pumpedfibre laser, where the input-end 194 has a high NA for efficientcoupling of pump light (commonly from a solid state laser source). Atthe output-end 196 of such a fibre laser, where the pump light may beabsorbed to a large degree, the main power carried by the fibre will beat the signal wavelength in the core region. Therefore, at theoutput-end, the air clad layer plays little or no effect and the voidsmay be fully or partially collapsed.

[0146] Optical fibres according to the present invention will oftencomprise Bragg gratings along a part of their length, for example forrealization of optical fibre lasers. These Bragg gratings may beintroduced by UV-writing of refractive index changes in the longitudinaldirection of the optical fibres.

[0147] An example of a preferred embodiment of an otical fibre accordingto the present invention that may be used for fibre laser applicationsis shown in FIG. 20. The figure shows schematically the cross-section ofa fibre that contains an active core region 200, typically realised bydoping of one or more rare-earth elements, such as for example Yb or Er.Surrounding the core region is an inner cladding comprising a backgroundmaterial 201 and a number of high- and/or low-index features 202.Surrounding the inner cladding region is an air-clad layer 203 thatfinally is surrounded by an overcladding region 204. Such a fibre, mayfor example be designed in silica materials, with the inner claddingfeatures 202 being voids. In a preferred embodiment, the optical fibreis used as a large-mode area, cladding pumped optical fibre laser forhigh-power applications. Using low-index features in the inner claddingwith a diameter of about 0.30 to 0.50 times an average, typical orrepresentative centre-to-centre spacing, Λ, between the inner claddingfeatures, single-mode operation at a signal wavelength, λ_(s), may beobtained in the core region, whereas the core as well as the innercladding region may be pumped with a pump light at a wavelength, λ_(p),being smaller than λ_(s). Typical values of λ_(p) are about 800 nm, 980nm, 1050 nm, and 1480 nm and typical values of λ_(s) are about 980 nm,1050 nm, 1300 nm, 1550 nm, such as from 1500 nm to 1640 nm. Even in thecase of a significant number and size of the inner cladding features, ahigh NA may be obtained using an air-clad layer when b values are in therange from 100 nm to 1000 nm (preferably smaller than 400 nm)—aspreviously described. Typically, the core region 200 will comprise oneor more index-raising dopants (that are introduced to improveincorporation of rare-earths into silica). It may therefore be preferredto fabricate the optical fibre using a background material 201 having arefractive index above that of pure silica, such as larger than 1.444 ata wavelength of 1.55 μm. This may for example be obtained by having abackground material 201 comprising Ge and/or Al. The core region mayalso comprise one or more co-dopants (apart from one or more (active)rare-earth dopants and any optional index-raising co-dopants) thatdecrease the refractive index, such as for example F and/or B. Regardingdimensions of a fibre, such as the one, shown in FIG. 20, typically theinner cladding region will have an outer diameter of about 60 μm-1001μm, for fibres with an outer diameter of about 125 μm of the outercladding region 204. For fibres having a larger outer diameter, theinner cladding may have an outer diameter up to 200 μm. Typically, theinner cladding features will be characterized by a typicalcentre-to-centre spacing, Λ, of more than 5 μm, and typically about 10μm, such as in the range from 8.0 μm to 12.0 μm. In order to avoidbending losses, but to maintain the fibre single mode in the core regionat the signal wavelength, the inner cladding features will typicallyhave a diameter in the range from 0.3 Λ to 0.6 Λ^(˜) The fibre may beemployed as part of an article being a fibre laser, where the articlecomprises one or more pump sources and external reflectors or thereflectors are formed directly in the fibre using one or more UV-inducedBragg gratings.

[0148] Optical fibres according to the present invention that are usedfor laser or amplifier applications may be pumped in various ways knownfrom standard fibre technology, such as end- and side-pumping.

[0149]FIG. 21 shows an optical microscope photograph of a preferredembodiment of an optical fibre according to the present invention thathas been fabricated. The fibre has NA of about 0.6 at wavelength about1.0 μm. This high NA is obtained by the use of very narrow bridgingregions in the air-clad region. The specific fibre has a b-value ofabout 400 nm. A number of fibres with different b-values have also beenfabricated and experimental characterizations have confirmed thatoptical fibres with b-values in the range from about 100 nm to 500 nmprovide NA of more than 0.5, typically more than 0.6.

[0150] From experimental work, it turns out that it may be of practicalrelevance that the b-value of a real optical fibre is larger than acertain size. This is for example the case in relation to cleavingand/or for splicing of the fibres. Typically, it is preferred that theb-value is not smaller than 100 nm, such as a b-value of not smallerthan 200 nm may be preferred.

[0151] The present inventors have further realized another designparameter that is of practical importance for fibres having an air-cladlayer. Through experimental work, the present inventors have realizedthat the thickness, T, of the air-clad layer 223 plays an important rolemechanically for cleaving of the fibres. The T-parameter is indicated inFIG. 22, for a fibre with a doped core region 220, an inner claddingregion 221, an air-clad region 223 and an outer cladding region 222. Ascleaving is usually performed by introducing some kind of scratch to theouter surface of the fibre, and having this scratch developing into acrack going through the fibre, it may be a disadvantage if the air-cladlayer has a too large thickness, T, such that the inner cladding regionbecomes mechanically isolated from the outer cladding. On the otherhand, the thickness, T, has to be of a certain size in order tooptically isolate the inner 221 and outer cladding 222. The presentinventors have realized that an optimum thickness of the air-clad layer223 is in the range from about 3.0 μm to about 10 μm. Since fibresaccording to the present invention may also be used for applicationswhere the air-clad layer after fibre fabrication is filled by e.g.polymers and/or other materials, other thickness values may be ofinterest in order to obtain a given volume. Hence, air-clad layers withthickness larger than 10 μm may also be of relevance.

[0152]FIG. 23 shows another example of a real fibre according to thepresent invention. FIG. 23a shows the core, the inner cladding region,the air-clad layer and part of the outer cladding (the core comprisesYb-doped silica that does not show different than the pure silica of theinner cladding in the microscope picture). The fibre has a b-value ofabout 300 nm and a NA of about 0.7 at wavelengths about 1.0 μm. Theoptical fibre has a larger T-value than the fibre in FIG. 21, and thefibre showed more difficult to cleave in agreement with theabove-described relation between T-value and cleave-ability.

[0153]FIG. 24 shows schematically another example of a fibre accordingto the present invention, which fibre may be used in endoscopes. Thefibre comprises an overcladding 240 and large number (typically morethan 100) of core elements 241 that are typically multi-mode andpassive. The core elements are separated from each other using air-cladlayers (or other types of low index layers) that provide a high NA forthe individual cores. In this manner, the fibre according to the presentinvention is capable of collecting a large amount of light due to thehigh NA of the individual cores—and these individual cores may be usedas pixels for image transfer through the fibre endoscope. To obtain thehigh NA, the air-clad layer is characterized with narrow bridgingwidths—as described throughout this patent application. Typically, thediameter of the individual cores is about 10 μm or larger.

[0154]FIG. 25 shows another example of a fibre according to the presentinvention. The fibre resembles the fibre schematically shown in FIG.20—having a high NA airclad layer 253 as disclosed in the presentapplication. The fibre, however, comprises a multitude of cores 250, 251(such as for example 7, 19, or 37) that are positioned such that adistance from a centre of a core to a centre of its nearest neighbouringcore is similar for all cores in the fibre. This type of fibre may beused as a high-brightness laser in a similar manner as described by Cheoet al. in IEEE Photonics Technology Letters, Vol. 13, no. 5, pp.439-441, May 2001. The fibre optionally comprises a number of features252 in the inner cladding region.

[0155]FIG. 26 shows yet another example of a fibre according to thepresent invention. The fibre comprises a high NA air-clad layer 263according to a main aspect of the present invention and a multitude ofcores 262 in an annular arrangement inside the air-clad layer. Otherarrangements; such as for example a polygonal-shaped arragement may alsobe preferred. The fibre further comprises a conventional overcladdingregion 264. The fibre may be used as a cladding-pumped device, where thepump light is propagating inside the core arrangement. This type of corearrangement may be preferred for example for improved couplingefficiency as described by Glas et al. Opt. Comm. 151, pp. 187-195,1998.

[0156] The present inventors have further realized that an improved typeof cladding pumped erbium-doped fibre amplifiers (EDFAs) may be obtainedthrough the use of the PBG effect for confining the core mode inside thefibre. Generally, when optical fields are confined through the use ofthe PBG effect, at least 4-5 periods of the periodically distributedcladding holes need to be used. As optical cladding pumped fibres mayhave relative large inner cladding regions a sufficient number ofperiods may well be included for confinement of the core mode. Theadvantages of using PBG confinement are secured. A first aspect is thatthe PBG structuring at the wavelength of the amplified mode may work asa mode-scrambling structure of air-holes at the wavelength of the pumpmode field, but without confining the pump distribution to the limitedcenter part of the PCF. Hence, in preferred embodiments, the presentinvention comprises air-clad fibres having periodically distributedfeatures in the inner cladding region that provide waveguidance by PBGeffect of light at signal wavelength.

[0157]FIG. 27-shows an example of an optical fibre according to thepresent invention that comprises an air-clad layer 270 for providing ahigh NA, and a number of periodically distributed features 271 in theinner cladding. The fibre further comprises a low-index feature 272 inthe core. The feature 272 may for example be a void or down-dopedsilica-glass. The features may optionally comprise active material, suchas a rare-earth-doped (RED) material, that provides amplification foroptical amplification or lasing. The fibre may also optionally comprisean active material in a region surrounding the feature 272.

[0158] Furthermore, we may obtain a better power conversion from pump tosignal because different overlap between RED material signal mode, andcladding pump distribution may be obtained. In a further aspect, the PBGguidance may be used to obtain high-power output from lasers andamplifiers operating in a higher-order mode (a non-gaussian modedistribution). This is possible because the PBG structure may bedesigned such that the core mode will only guide in a higher-order modeand all other modes will be leaky because they are placed outside thebandgap. The PBG guidance for the amplified mode may also find relevantuse in applications where amplifiers with special dispersion propertiesare needed (for pulse spreading or pulse compression).

[0159] In yet a further aspect, the PBG guidance may be used to enhancespecific parts of the amplifier spectrum. Here, we may place the bandgapedge at a frequency within the emission spectrum of the rare-earth ion.For the part of the RED-emission spectrum which is inside the bandgap,the core mode is well confined, whereas the spectral components whichare outside the bandgap are less well confined, and consequently, thetwo ranges undergo different amplification. This property may be used tospectrally shape new high-power amplifiers and to fabricate lasers withspecial emission wavelengths, through the strong modeselection/discrimination made possible by the PBG effect. A schematicillustration of this aspect is presented in FIG. 28.

[0160] Fibres according to the present invention may be fabricated usingtechniques that are well known in the area of micro-structured fibres.For example, the air-clad layer and optional features in the innercladding region may be realised using a stack- and draw method thatemploys capillary tubes and rods. This method has been well described inliterature, see e.g. U.S. Pat. No. 5,907,652 and U.S. Pat. No.5,802,236. Fibres according to the present invention that arenon-uniform in the longitudinal direction, may be realized using varioustypes of post-processing steps after fibre drawing, such asheat-treatment, stretching, pressurizing or vacuum-treatment of thevoids in the fibres, introduction of materials into the voids orcombinations of these steps.

[0161] Specifically for fabricating fibres such as the fibre in FIG. 24,it may be advantageous to draw the fibre in multiple steps, where asingle core and its surrounding air-clad layer is fabricated by in afirst step assembling a preform comprising a single solid rod surroundedby a layer of tubes of smaller diameter than the rod. This preformhaving a diameter of typically 10 mm to 50 mm may be drawn into a numberof first canes—typically of diameter 1 mm to 5 mm. A second preform maythen be fabricated by stacking a number of first canes together and thispreform may optionally be over-cladded and drawn directly into fibre, orthe second preform may be drawn to a number of second canes that againmay be stacked together and overcladded to form a third preform that maybe drawn into fibre.

[0162]FIG. 29a shows experimental results of measured NA for twodifferent air-clad fibres. Each of the fibres has an air-clad regionwith a design as shown schematically in FIG. 5b, but the fibres havedifferent b-values of 420 nm and 950 nm. In the figure, measured NAvalues are indicated by points, whereas the lines indicated simulated NAof the fibres. A very good agreement for the fibres is observed. Thefigure shows that the NA of the fibre with bridging regions of minimumwidth, b, of about 420 nm is significantly higher than for the fibrewith b of about 950 nm. A microscope picture of a part of the fibre withb=420 nm is shown in FIG. 29b. The air-clad layer comprises a singlering of air holes and preferably the width of the air holes in radialdirection from the centre of the fibre is in the range of 5 μm to 15 μm.It is here desired to keep the radial width—and thereby the bridgingregions—long enough to ensure the optical properties in terms of highNA, while the radial width is short enough to ensure good mechanicalproperties in terms of cleaving of the fibre and/or handling strength.Further issues to consider for the width, may be heat transfer from theinner parts of the fibre to the outside. For high power applications, itmay be desirable to have a limited radial width of the air-clad regionin order to avoid thermal isolation. Accordingly, it may be preferred tohave a high number of bridging regions equivalently a high number oflow-index features in the outer cladding—to provide sufficient heattransfer. In preferred embodiments, a smallest cross-sectional distancefrom two neighbouring bridging regions is, therefore, kept above a firstsize to ensure optical isolation (typically a distance of about three tofive times an operating wavelength is sufficient to ensure isolation ofthe individual bridges), while at the same time being lower than asecond size to ensure a sufficiently large number of bridges (forexample a second size of about ten times an operating wavelength). Otherfirst and second sizes may, however, be preferred. Typically, the rangeof the second size may vary significantly, such that a second size ofseveral tens times an operational wavelength may be preferred.

[0163]FIG. 29a further shows how the NA is varying with wavelength. Thepresent inventors have realized that the NA and its variation areprimarily determined by the wavelength relative to the b-parameter. Todemonstrate this in more detail, FIG. 30 shows the NA as a function ofwavelength divided by b for both experimentally obtained results andsimulations. As for FIG. 29, the results in FIG. 30 are for a fibresolely comprising pure silica and air holes in the air-clad region. Thefigure shows a very clear relation between NA and wavelength divided byb, λ/b—and the experimentally observed results are confirmed bynumerical simulations. Hence, FIG. 30 may be used to design fibres witha certain NA at a given wavelength. From FIG. 30 it is found that inorder to obtain an NA of about 0.3or higher, b should be smaller than λ.For an NA of about 0.4 or higher, b should be smaller than approximately0.8λ, and for NA higher than 0.5, b should be smaller than approximately0.6λ. and for NA higher than 0.6, b should be smaller than approximately0.45λ The figure also illustrates that extremely higher NA of more than0.7 is feasible using b of smaller than approximately 0.3λ. Hence, foran applications where a fibre according to the present invention is usedas a cladding pumped device (for example a laser or an amplifier) with apumping wave-length of about 980 nm and a NA of about 0.5 is desired, bshould be designed to be about or smaller than 560 nm.

[0164] For various applications, it may be desired to obtain the largestpossible bridging regions for a given NA. This may for example be forimproved mechanical properties such as for fibre strength or cleaving,or it may be for improved heat transfer as discussed previously. Inorder to increase the bridging width for a given NA, the presentinventors have realised that it is an advantage to have an indexdifference, Δ_(clad), between the material of the bridging regions andthe background material of the inner cladding. FIG. 31 showsschematically an example of an improved fibre according to the presentinvention, where the outer cladding region comprises a backgroundmaterial 310 having a lower refractive index than the refractive indexof the background material 311 in the inner cladding region. This indexdifference may for example be realised using silica-doping techniques,where the inner cladding region comprises Ge-doped silica glass and theouter cladding region comprises un-doped silica glass. Naturally, othermanners of realising this index difference may be thought of, forexample using F-doped glass in the outer cladding as Flour decreases therefractive index compared to pure silica. The index difference,Δ_(clad), is also schematically illustrated in FIG. 32 that shows aschematic example of the refractive index profile in one directionthrough the cross-section of a fibre according the present invention.While FIG. 30 showed the NA as a function of λ/b for no index differencebetween the refractive index of the background material in the inner andouter cladding, Δ_(clad)=0%, FIG. 33 shows how the NA may be increasedby increasing Δ_(clad). The figure shows simulations of the NA forsilica-based fibres where Δ_(clad) is varied from 0% to 4%. As seen fromthe figure, a significant increase in NA may be realised using non-zeroΔ_(clad). As an example, the previously discussed fibre device operatingat a pump wavelength of 980 nm and an NA of 0.5, a b-value of about 650nm may be realised as compared to b=560 nm for the fibre withΔ_(clad)=0% (NA of 0.5 occurs for λ/b of about 1.50 and 1.75 forΔ_(clad)=1% and 0%; respectively). As another example, fibres with an NAof about 0.5 may be realized for b of about λ for Δ_(clad) about 3%.Further, fibres with NA of higher than 0.8 may be realised for λ/b ofabout 3.0 or larger. The ideas of increasing the NA using non-zeroΔ_(clad), may be utilized for all types of high NA fibres comprisinglow-index features 314, 324 in the outer cladding region and is notlimited to the two example shown in FIG. 31 and 32 having an active core313, 320 and inner cladding features 312, 322 in the inner claddingregion 321. Optionally, the overcladding region 315, 325 may comprise abackground material being different than the background material 310.While it is the refractive index difference between the material in thebridging regions and the background material in the inner cladding thatprovides the improved NA properties, the radial width of the outercladding comprising a low-index background material 317 may not becritical. It may be preferred that the radial width 317 is larger thanthe radial width of a low-index feature in the outer cladding 316. Sucha relation could occur for a fibre being fabricated using the stack andpull process (as described for example in U.S. Pat. No. 5,907,652) wherethe outer cladding region is realised using silica capillary tubeshaving a lower refractive index than the rod for realising the innercladding region (and the core). The inner cladding region and the coremay naturally also be fabricated by various combinations of tubes and/orrods.

[0165] A further advantage of using a non-zero Δ_(clad) relates tocladding pumped fibres having a large active core. FIG. 32 shows theindex difference, Δ_(core), between an active core 320 and thebackground material of the inner cladding 321. In a preferredembodiment, the fibre uses inner cladding features 322 to confine lightin the core—either using modified total internal reflection (M-TIR) asfor example described in WO 9900685 or using PBG effects as previouslydescribed. In the case of M-TIR, it is preferred that the indexdifference, Δ_(core), is as low as possible to avoid multimode operationat the signal wavelength for large core sizes. In preferred embodiments,Δ_(core) is about zero and in other preferred embodiments, Δ_(core) maybe negative. A low (or negative) Δ_(core) allows use of very largeactive cores of more than 15 μm in diameter while single mode operationfor at least the signal wavelength is obtained. As dopants for realizingactive cores (such as for example Er, Yb, or other rare earth elementsor combinations of these) and optional co-dopants (such as for exampleAl, Ge, and/or La) may increase the refractive index compared to puresilica, it is preferred to use a background material in the innercladding region with a raised refractive index—for example usingGe-doped silica. While this may provide □core of about zero, a Δ_(clad)value above zero may be obtained at the same time. Hence, in a preferredembodiment, the present invention provides an optical fibre laser oramplifier for visible or near-infrared wavelengths comprising an activecore comprising Er and/or Yb having a diameter of more than 15 μm, andinner cladding region comprising a Ge-doped silica background materialand number of voids having d/Λ of about 0.35 or larger (providing M-TIRin the core for the signal), and an air-clad region comprising abackground material (for example pure silica) with a refractive indexbeing lower than that of the inner cladding background material andlarge voids providing a b-value of less than 1.0 μm, preferably less ⅔times a free-space pump wavelength (thereby realising an NA of about 0.5or higher).

1. An article comprising an optical fibre, the fibre comprising at leastone core surrounded by a first outer cladding region, the first outercladding region being surrounded by a second outer cladding region, thefirst outer cladding region in the cross-section comprising a number offirst outer cladding features having a lower refractive index than anymaterial surrounding the first outer cladding features, wherein for aplurality of said first outer cladding features, the minimum distancebetween two nearest neighbouring first outer cladding features issmaller than 1.0 μm or smaller than an optical wavelength of lightguided through the fibre when in use.
 2. An article according to claim1, wherein the core is surrounded by an outer cladding, the outercladding comprising the first and the second outer cladding regions withthe first outer cladding region arranged between the core and the secondouter cladding region.
 3. An article according to claim 1 or 2, whereinthe first outer cladding region in the cross section has an innerdiameter or inner cross-sectional dimension being larger than or equalto 15 μm.
 4. An article according to claim 3, wherein the inner diameteror inner cross-sectional dimension of the first outer cladding region islarger than or equal to 20 μm.
 5. An article according to claim 1 or 2,wherein the inner diameter or inner cross-sectional dimension of thefirst outer cladding region is in the range from 80 μm to 125 μm or inthe range from 125 μm to 350 μm.
 6. An article according to claim 1,wherein the fibre comprises a number of cores with a plurality of saidnumber of cores each being surrounded by a corresponding first outercladding region, the cores and the first outer cladding regions beingsurrounded by the second outer cladding region, each of the first outercladding regions in the cross-section comprising a number of first outercladding features having a lower refractive index than any materialsurrounding the first outer cladding features, wherein for a pluralityof the first outer cladding features of each of said first outercladding regions, the minimum distance between two nearest neighbouringfirst outer cladding features is smaller than 1.0 μmm or smaller than anoptical wavelength of light guided through the fibre when in use.
 7. Anarticle comprising an optical fibre, the fibre comprising a number ofcores with a plurality of said number of cores each being surrounded bya corresponding first outer cladding region, the cores and the firstouter cladding regions being surrounded by a second outer claddingregion, each of the first outer cladding regions in the cross-sectioncomprising a number of first outer cladding features having a lowerrefractive index than any material surrounding the first outer claddingfeatures, wherein for a plurality of the first outer cladding featuresof each of said first outer cladding regions, the minimum distancebetween two nearest neighbouring first outer cladding features issmaller than 1.0 μm or smaller than an optical wavelength of lightguided through the fibre when in use.
 8. An article according to claim 6or 7, wherein each of said number of cores are surrounded by a firstouter cladding region.
 9. An article according to any of the claims 6-8,wherein at least part of the plurality of cores each being surrounded bya corresponding first outer cladding region are arranged so that thefirst outer cladding regions of two neighbouring cores share a number ofsaid first outer cladding features.
 10. An article according to claim 9,wherein all of the plurality of cores each being surrounded by a firstouter cladding region are arranged so that the first outer claddingregions of two neighbouring cores share a number of said first outercladding features.
 11. An article according to any of the claims 6-10,wherein a number or all of the first outer cladding regions in the crosssection have an inner diameter or inner cross-sectional dimension beingin the range of 5-100 μm.
 12. An article according to any of the claims6-11, wherein a number or all of the first outer cladding regions in thecross section have an inner diameter or inner cross-sectional dimensionbeing in the range of 30-60 μm.
 13. An article according to any of theclaims 6-12, wherein said plurality of cores each being surrounded by afirst outer cladding region comprises at least 20 cores.
 14. An articleaccording to claims 13, wherein said plurality of cores each beingsurrounded by a first outer cladding region comprises at least 100cores.
 15. An article according to claims 14, wherein said plurality ofcores each being surrounded by a first outer cladding region comprisesat least 1000 cores.
 16. An article according to claims 15, wherein saidplurality of cores each being surrounded by a first outer claddingregion comprises at least 3000 cores.
 17. An article according to any ofthe claims 1-16, wherein the second outer cladding region is part of anouter cladding, which further comprises third and fourth outer claddingregions with the third outer cladding region arranged between the secondand the fourth outer cladding region, the third outer cladding region inthe cross-section comprising a number of third outer cladding featureshaving a lower refractive index than any material surrounding the thirdouter cladding features, wherein for a plurality of said third outercladding features, the minimum distance between two nearest neighbouringthird outer cladding features is smaller than 1.0 μm or smaller than anoptical wavelength of light guided through the fibre when in use.
 18. Anarticle according to any of the preceding claims, wherein for aplurality of said first and/or third outer cladding features the minimumdistance between two nearest neighbouring outer cladding features issmaller than 0.5 μm.
 19. An article according to any of the precedingclaims, wherein for a plurality of said first and/or third outercladding features the minimum distance between two nearest neighbouringouter cladding features is smaller than 0.4 μm.
 20. An article accordingto any of the preceding claims, wherein for a plurality of said firstand/or third outer cladding features the minimum distance between twonearest neighbouring outer cladding features is smaller than 0.3 μm. 21.An article according to any of the preceding claims, wherein for aplurality of said first and/or third outer cladding features the minimumdistance between two nearest neighbouring outer cladding features issmaller than 0.2 μm.
 22. An article according to any of the claims 1-21,wherein for a plurality of said first outer cladding features, theminimum distance between two nearest neighbouring first outer claddingfeatures is smaller than a shortest optical wavelength of light guidedthrough the fibre when in use.
 23. An article according to any of theclaims 17-22, wherein for a plurality of said third outer claddingfeatures, the minimum distance between two nearest neighbouring thirdouter cladding features is smaller than a shortest optical wavelength oflight guided through the fibre when in use.
 24. An article according toany of the claims 1-10 or 13-23, wherein the core(s) has/have across-sectional dimension larger than 25 μm, such as larger than 50 μm,such as larger than 75 μm, such as larger than 100 μm, thereby causingthe fibre to guide light in multiple modes within the core.
 25. Anarticle according to any of the claims 1-24, wherein one or more of thecores is/are surrounded by an inner cladding region having an effectiverefractive index with a lower value than the effective refractive indexof the core being surrounded, said inner cladding regions being part ofan inner cladding.
 26. An article according to claim 25, wherein eachinner cladding region is surrounded by a corresponding first outercladding region.
 27. An article according to any of the precedingclaims, wherein for one or more first outer cladding regions the firstouter cladding features occupy 45% or more of the cross-sectional areaof the first outer cladding region.
 28. An article according to claim27, wherein for all first outer cladding regions the first outercladding features occupy 45% or more of the cross-sectional area of thefirst outer cladding region.
 29. An article according to claim 27 or 28,wherein the first outer cladding features occupy at least 50%, such asat least 60%, or such as at least 70% of the cross-sectional area of thefirst outer cladding.
 30. An article according to any of the claims17-29, wherein the third outer cladding features occupy 45% or more ofthe third outer cladding.
 31. An article according to claim 30, whereinthe third outer cladding features occupy at least 50%, such as at least60%, or such as at least 70% of the cross-sectional area of the thirdouter cladding.
 32. An article according to any of the claims 25-31,wherein the inner cladding or one or more of the inner cladding regionsin the cross-section comprise one single inner cladding feature.
 33. Anarticle according to any of the claims 25-31, wherein the inner claddingor one or more of the inner cladding regions in the cross-sectioncomprise at least two inner cladding features.
 34. An article accordingto claim 33, wherein the inner cladding or each inner cladding region inthe cross-section comprises less than 10 inner cladding features.
 35. Anarticle according to claim 33 or 34, wherein the inner cladding featuresare placed in a non-periodic manner.
 36. An article according to any ofthe claims 1-5 or 17-25 or 27-31, wherein the optical fibre comprises atleast two cores being surrounded by a common first outer claddingregion.
 37. An article according to claim 36, wherein the optical fibrecomprises at least seven cores, at least 19 cores, or at least 37 coresbeing surrounded by a common first outer cladding region.
 38. An articleaccording to claim 36 or 37, wherein the optical fibre comprises aplurality of cores in a substantially annular arrangement, saidplurality of cores being surrounded by the common first outer claddingregion.
 39. An article according to claim 38, wherein said plurality ofsubstantially annular arranged cores are arranged in an inner claddingso that a major part of the inner cladding are surrounded by thesubstantially annular arranged cores, said inner cladding having aneffective refractive index with a lower value than the effectiverefractive index of each of said substantially annular arranged cores.40. An article according to any of the claims 36-39, wherein thedistance between one set of two nearest neighbouring cores issubstantially identical to the distance between other sets of twonearest neighbouring cores.
 41. An article according to any of claims36-40, wherein each of said cores being surrounded by a common firstouter cladding region is surrounded by an inner cladding region havingan effective refractive index with a lower value than the effectiverefractive index of the core being surrounded, said inner claddingregions being part of an inner cladding being surrounded by the commonfirst outer cladding region.
 42. An article according to any of theclaims 36-41, wherein the inner cladding in the cross-section comprisesa number of inner cladding features.
 43. An article according to claim42, wherein the inner cladding comprises at least two inner claddingfeatures.
 44. An article according to claim 43, wherein the innercladding comprises at least 10 inner cladding features.
 45. An articleaccording to any of claims 42-44, wherein at least part of the innercladding features are arranged so that a for an inner cladding regionsurrounding one of said cores, said inner cladding region comprisesseveral inner cladding features.
 46. An article according to claim 45,wherein the inner cladding features of an inner cladding region areperiodically arranged.
 47. An article according to any of the claims 1-5or 17-24 or 27-31, wherein the optical fibre comprises a core beingsurrounded by an inner cladding having a number of inner claddingfeatures being periodically distributed within said inner cladding, saidinner cladding being surrounded by the first outer cladding region. 48.An article according to claim 47, wherein the core comprises a corefeature having a lower refractive index than the refractive index of thecore material surrounding the core feature.
 49. An article according toclaim 47 or 48, wherein the periodically arrangement of the innercladding features comprises at least 4 or 5 periods in a radialdirection from the centre of the inner cladding.
 50. An articleaccording to any of the preceding claims, wherein the first outercladding features are placed in a non-circular symmetric manner.
 51. Anarticle according to any of the preceding claims, wherein the firstouter cladding region in the cross section has a substantially hexagonallike shape.
 52. An article according to any of the preceding claims,wherein first outer cladding features of different size are present. 53.An article according to any of the preceding claims, wherein the firstand/or third outer cladding features and/or the inner cladding featuresare voids.
 54. An article according to any of the preceding claims,wherein the first and/or third outer cladding features and/or the innercladding features are filled with vacuum, air, a gas, a liquid or apolymer or a combination thereof.
 55. An article according to any of theclaims 25-54, wherein the core(s) surrounded by an inner cladding regionhas/have a cross-sectional dimension smaller than 10 μm.
 56. An articleaccording to any of the preceding claims, wherein the core(s)comprises/comprise at least one member of the group consisting of Ge,Al, P, Sn and B.
 57. An article according to any of the precedingclaims, wherein the core(s) comprises/comprise at least one rare earth,such as Er, Yb, Nd, La, Ho, Dy and/or Tm.
 58. An article according toclaim 57, wherein the fibre has a long period grating or a fibre Bragggrating along at least a part of the fibre length.
 59. An articleaccording to any of the preceding claims, wherein the fibre comprises abackground material being silica, chalcogenide, or another type ofglass.
 60. An article according to any of the claims 1-58, wherein thefibre comprises a background material being polymer.
 61. An articleaccording to any of the claims 25-60, wherein the article is a claddingpumped fibre laser or amplifier.
 62. An article according to any of theclaims 25-61, wherein the article is a cladding pumped fibre laser oramplifier comprising a pump radiation source and a length of saidoptical fibre.
 63. An article according to claim 22 or 23 and claim 62,wherein said shortest optical wavelength is determined by the wavelengthof said pump radiation source.
 64. An article according to any of thepreceding claims, wherein said optical fibre in the cross-section has anon-uniform shape of the inner cladding region along the fibre length.65. An article according to any of the preceding claims, wherein thefirst and/or third outer cladding features are periodically distributed.66. An article according to any of the claims 25-65, wherein the innercladding region acts as a reservoir for light generated in the core. 67.An article according to any of the claims 25-66, wherein the opticalfibre comprises at least two cores.
 68. An article according to any ofthe claims 67, wherein said at least two cores are adapted for highpower broadband amplification.
 69. An article according to any of theclaims 32-68, wherein the fibre core material is the same as thebackground material of the inner cladding region.
 70. An articleaccording to any of the claims 25-69, wherein said optical fibre isoptically pumped bi-directionally.
 71. An article according to any ofthe preceding claims, wherein for a majority or all of said first outercladding features, the minimum distance between two nearest neighbouringouter cladding features is smaller than 1.0 μm.
 72. An article accordingto claim 71, wherein for a majority or all of said first outer claddingfeatures, the minimum distance between two nearest neighbouring outercladding features is smaller than 0.5 μm, such as smaller than 0.4 μm,such as smaller than 0.3 μm, or such as smaller than 0.2 μm.
 73. Anarticle according to any of the preceding claims, wherein for a majorityor all of said first outer cladding features, the minimum distancebetween two nearest neighbouring outer cladding features is smaller thanan optical wavelength of light guided through the fibre when in use. 74.An article according to any of the preceding claims, wherein for amajority or all of said first outer cladding features, the minimumdistance between two nearest neighbouring outer cladding features issmaller than a shortest optical wavelength of light guided through thefibre when in use.
 75. An article according to any of the claims 17-74,wherein for a majority or all of said third outer cladding, features,the minimum distance between two nearest neighbouring outer claddingfeatures is smaller than 1.0 μm.
 76. An article according to claim 75,wherein for a majority or all of said third outer cladding features, theminimum distance between two nearest neighbouring outer claddingfeatures is smaller than 0.5 μm, such as smaller than 0.4 μm, such assmaller than 0.3 μm, or such as smaller than 0.2 μm.
 77. An articleaccording to any of the claims 17-76, wherein for a majority or all ofsaid third outer cladding features, the minimum distance between twonearest neighbouring outer cladding features is smaller than an opticalwavelength of light guided through the fibre when in use.
 78. An articleaccording to any of the claims 17-77, wherein for a majority or all ofsaid third outer cladding features, the minimum distance between twonearest neighbouring outer cladding features is smaller than a shortestoptical wavelength of light guided through the fibre when in use.
 79. Anarticle according to any of the preceding claims, wherein the outercladding has an effective refractive index with a value lower than theeffective refractive index of any of the cores.
 80. An article accordingto any of the claims 25-79, wherein the outer cladding has an effectiverefractive index no, with a value lower than the effective refractiveindex of the inner cladding or any of the inner cladding regions.
 81. Anarticle according to any of the claims 17-80, wherein the second outercladding region is made of a homogeneous material.
 82. An articleaccording to any of the preceding claims, wherein the largestcross-sectional dimension of the first outer cladding features are equalto or below 10
 83. An article according to any of the preceding claims,wherein the largest cross-sectional dimension of the first outercladding features are equal to or below 3 μm.
 84. An article accordingto claim 25 or 26 and any of claims 27-31 or 33-83, wherein the innercladding comprises a background material and a plurality of features,said features having a refractive index being higher and/or lower thanthe refractive index of the background material.
 85. An articleaccording to claim 84, wherein the inner cladding features have a lowerindex than the refractive index of the background material.
 86. Anarticle according to claim 84 or 85, wherein the inner cladding featuresare voids.
 87. An article according to any of claims 81-86, wherein theinner cladding features are filled with vacuum, air, a gas, a liquid ora polymer or a combination thereof.
 88. An article according to any ofclaims 81-87, wherein the inner cladding features have a lower indexthan the refractive index of the background material, and the innercladding features have a cross-sectional diameter or dimension in therange of 0.3-0.6, preferably 0.3-0.5 times, more preferably 0.2-0.3times, and most perferably 0.22-0.28 times a centre-to-centre spacingbetween neighbouring inner cladding features.
 89. An article accordingto claim 88, wherein the centre-to-centre spacing between neighbouringinner cladding features is an average centre-to-centre spacing.
 90. Anarticle according to claim 88 or 89, wherein the centre-to-centrespacing is larger than 5 μm, in the range of 8-12 μm, or about 10 μm.91. An article according to any of claims 81-90, wherein the corecomprises one or more dopants for raising or lowering the refractiveindex above the refractive index of the background material of the innercladding.
 92. An article according to any of claims 81-91, wherein theouter diameter of the inner cladding is within the range from 60-400 μmor within the range from 200-400 μm.
 93. An article according to any ofthe preceding claims, wherein the optical fibre has a length with afirst end and a second end, and wherein the cross-sectional area of thefirst outer cladding features in the first end are larger than anycross-sectional area of first outer cladding features in the second end.94. An article according to claim 93, wherein the second end comprisesno first outer cladding features, or wherein the first outer claddingfeatures are fully collapsed in the second end.
 95. An article accordingto any of the claims 17-94, wherein the optical fibre has a length witha first end and a second end, and wherein the cross-sectional area ofthe third outer cladding features in the first end are larger than anycross-sectional area of third outer cladding features in the second end.96. An article according to claim 95, wherein the second end comprisesno third outer cladding features, or wherein the third outer claddingfeatures are fully collapsed in the second end.
 97. An article accordingto any of the preceding claims, wherein the first outer claddingfeatures are elongate features extending in a fibre axial direction. 98.An article according to any of the claims 17-97, wherein the third outercladding features are elongate features extending in a fibre axialdirection.
 99. An article according to any of the claims 32-98, whereinthe inner cladding features are elongate features extending in a fibreaxial direction.
 100. An article according to any of the precedingclaims, wherein the background material surrounding the first outercladding features or the bridging material fulfilling the area betweenneighbouring first outer cladding features has a lower refractive indexthan the refractive index of the background material of the innercladding.
 101. An article according to claim 100, wherein the backgroundmaterial surrounding the first outer cladding features or the bridgingmaterial fulfilling the area between neighbouring first outer claddingfeatures has a lower refractive index being less than 4% lower than therefractive index of the background material of the inner cladding, suchas less than 3% lower, such as 2% lower, such as 1% lower.
 102. Anarticle according to claim 100 or 101, wherein the core has a refractiveindex being less than 1.0% different from the refractive index of thebackground material of the inner cladding, such as less than 0.5%different, such as less than 0.2% different, such as less than 0.1%different, such as less than 0.05% different.
 103. An article accordingto claim 102, wherein the core has a refractive index being equal to orhigher than the refractive index of the background material of the innercladding.
 104. An article according to claim 102, wherein the core has arefractive index being equal to or lower than the refractive index ofthe background material of the inner cladding.
 105. An article accordingto any of the claims 6-100, wherein the article is an endoscope.
 106. Anoptical fibre for guiding light of at least one predeterminedwavelength, the optical fiber having a longitudinal direction and across-section perpendicular thereto, the optical fibre comprising: (a)at least one core region (52, 180, 241, 250, 251, 262); (b) a claddingregion, said cladding region comprising: an outer cladding, said outercladding comprising: (i) at least one a first outer cladding region,said first outer cladding region comprising a first outer claddingbackground material and a plurality of first outer cladding features(50, 51, 181, 242), said first outer cladding features having a lowerrefractive index than said first outer cladding background material, andsurrounding said at least one core region, and (ii) at least one furtherouter cladding region (54, 171, 172, 173, 182, 240), each of said atleast one further outer cladding regions comprising a further outercladding background material, and surrounding said first outer claddingregion, wherein for a plurality of said first outer cladding features,two nearest neighbouring first outer cladding features have a minimumdistance smaller than the wave-length of said light of at least onepredetermined wave-length.
 107. The optical fiber according to claim 106further comprising an inner cladding region (53, 102, 201), said innercladding region comprising an inner cladding background material. 108.An optical fibre according to claims 106 or 107 said inner claddingregion comprising at least one inner cladding feature (101, 202). 109.An optical fibre according to claim 106, wherein the core is surroundedby an outer cladding, the outer cladding comprising the first and afurther outer cladding regions with the first outer cladding regionarranged between the core and the further outer cladding region.
 110. Anoptical fibre according to any one of claims 106-109 wherein the firstouter cladding region in the cross section has an inner diameter orinner cross-sectional dimension being larger than or equal to 15 μm.111. An optical fibre according to any one of claims 106-110 wherein theinner diameter or inner cross-sectional dimension of the first outercladding region is larger than or equal to 20 μm.
 112. An optical fibreaccording to any one of claims 106-112 wherein the inner diameter orinner cross-sectional dimension of the first outer cladding region is inthe range from 80 μm to 125 μm or in the range from 125 μm to 350 μm.113. An optical fibre according to any one of claims 106-112 whereineach of said at least one core regions are surrounded by a correspondingfirst outer cladding region.
 114. An optical fibre according to any oneof claims 106-113 wherein each of said at least one core regions aresurrounded by a common first outer cladding region.
 115. An opticalfibre according to any one of claims 106-114 wherein at least part ofthe plurality of cores each being surrounded by a corresponding firstouter cladding region are arranged so that the first outer claddingregions of two neighbouring cores share a number of said first outercladding features.
 116. An optical fibre according to any one of claims106-115 wherein all of the plurality of cores each being surrounded by afirst outer cladding region are arranged so that the first outercladding regions of two neighbouring cores share a number of said firstouter cladding features.
 117. An optical fibre according to any one ofclaims 106-116 wherein a number or all of the first outer claddingregions in the cross section have an inner diameter or innercross-sectional dimension being in the range of 5-100 μm.
 118. Anoptical fibre according to any one of claims 106-117 wherein a number orall of the first outer cladding regions in the cross section have aninner diameter or inner cross-sectional dimension being in the range of30-60 Wu.
 119. An optical fibre according to any one of claims 106-118wherein said plurality of cores each being surrounded by a first outercladding region comprises at least 20 cores, preferably at least 100cores, more preferred at least 1000 cores, in particular at least 3000cores.
 120. An optical fibre according to any one of claims 106-119wherein for a plurality of said first outer cladding features, theminimum distance between two nearest neighbouring outer claddingfeatures is smaller than 1.0 μm, preferably smaller than 0.8, morepreferably smaller than 0.7, still more preferably smaller than 0.6,most preferably smaller than 0.5 μm, particularly smaller than 0.4 μm,more particularly smaller that 0.3 μm, most particularly smaller than0.2 μm.
 121. An optical fibre according to any one of claims 106-120guiding at leat two predetermined wavelength wherein for a plurality ofsaid first outer cladding features, the minimum distance between twonearest neighbouring first outer cladding features is smaller than ashortest optical wavelength of light guided through the fibre.
 122. Anoptical fibre according to any one of claims 106-121 wherein the core(s)has/have a cross-sectional dimension larger than 25 μm, such as largerthan 50 μm, such as larger than 75 μm, such as larger than 100 μm,thereby causing the fibre to guide light in multiple modes within thecore.
 123. An optical fibre according to any one of claims 106-122wherein one or more of the cores is/are surrounded by an inner claddingregion having an effective refractive index with a lower value than theeffective refractive index of the core being surrounded, said innercladding regions being part of an inner cladding.
 124. An optical fibreaccording to any one of claims 106-123 wherein each inner claddingregion is surrounded by a corresponding first outer cladding region.125. An optical fibre according to any one of claims 106-124 wherein forone or more first outer cladding regions the first outer claddingfeatures occupy 45% or more of the cross-sectional area of the firstouter cladding region.
 126. An optical fibre according to any one ofclaims 106-125 wherein for all first outer cladding regions the firstouter cladding features occupy 45% or more of the cross-sectional areaof the first outer cladding region.
 127. An optical fibre according toany one of claims 106-126 wherein the first outer cladding featuresoccupy at least 50%, such as at least 60%, or such as at least 70% ofthe cross-sectional area of the first outer cladding.
 128. An opticalfibre according to any one of claims 106-127 wherein the inner claddingor one or more of the inner cladding regions in the cross-sectioncomprise one single inner cladding feature.
 129. An optical fibreaccording to any one of claims 106-128 wherein the inner cladding or oneor more of the inner cladding regions in the cross-section comprise atleast two inner cladding features.
 130. An optical fibre according toany one of claims 106-129 wherein the inner cladding or each innercladding region in the cross-section comprises less than 10 innercladding features.
 131. An optical fibre according to any one of claims106-130 wherein the inner cladding features are placed in a non-periodicmanner.
 132. An optical fibre according to any one of claims 106-131wherein the optical fibre comprises at least two cores being surroundedby a common first outer cladding region.
 133. An optical fibre accordingto any one of claims 106-132 wherein the optical fibre comprises atleast seven cores, at least 19 cores, or at least 37 cores beingsurrounded by a common first outer cladding region.
 134. An opticalfibre according to any one of claims 106-133 wherein the optical fibrecomprises a plurality of cores in a substantially annular arrangement,said plurality of cores being surrounded by the common first outercladding region.
 135. An optical fibre according to any one of claims106-134 wherein the optical fibre comprises a core being surrounded byan inner cladding having a number of inner cladding features beingperiodically distributed within said inner cladding, said inner claddingbeing surrounded by the first outer cladding region.
 136. An opticalfibre according to any one of claims 106-135 wherein the core comprisesa core feature having a lower refractive index than the refractive indexof the core material surrounding the core feature.
 137. An optical fibreaccording to any one of claims 106-136 wherein the periodicallyarrangement of the inner cladding features comprises at least 4 or 5periods in a radial direction from the centre of the inner cladding.138. An optical fibre according to any one of claims 106-137 wherein thefirst outer cladding features are placed in a non-circular symmetricmanner.
 139. An optical fibre according to any one of claims 106-138wherein the first outer cladding region in the cross section has asubstantially hexagonal like shape.
 140. An optical fibre according toany one of claims 106-139 wherein first outer cladding features ofdifferent size are present.
 141. An optical fibre according to any oneof claims 106-140 wherein the first outer cladding features are voids.142. An optical fibre according to any one of claims 106-141 wherein thefirst outer cladding features are filled with vacuum, air, a gas, aliquid or a polymer or a combination thereof.
 143. An optical fibreaccording to any one of claims 106-142 wherein the core(s) surrounded byan inner cladding region has/have a cross-sectional dimension smallerthan 10 μm.
 144. An optical fibre according to any one of claims 106-143wherein the core(s) comprises/comprise at least one member of the groupconsisting of Ge, Al, P, Sn and B.
 145. An optical fibre according toany one of claims 106-144 wherein the core(s) comprises/comprise atleast one rare earth, such as Er, Yb, Nd, La, Ho, Dy and/or Tm.
 146. Anoptical fibre according to any one of claims 106-145 wherein the fibrehas a long period grating or a fibre Bragg grating along at least a partof the fibre length.
 147. An optical fibre according to any one ofclaims 106-146 wherein the fibre comprises a background material beingsilica, chalcogenide, or another type of glass.
 148. An optical fibreaccording to any one of claims 106-147 wherein the fibre comprises abackground material being polymer.
 149. An optical fibre according toany one of claims 106-148 wherein the optical fibre is a cladding pumpedfibre laser or amplifier.
 150. An optical fibre according to any one ofclaims 106-149 wherein the optical fibre is a cladding pumped fibrelaser or amplifier comprising a pump radiation source and a length ofsaid optical fibre.
 151. An optical fibre according to any one of claims106-150 wherein said predetermined wavelength is determined by thewavelength of said pump radiation source.
 152. An optical fibreaccording to any one of claims 106-151 wherein said optical fibre in thecross-section has a non-uniform shape of the inner cladding region alongthe fibre length.
 153. An optical fibre according to any one of claims106-153 wherein the first outer cladding features are periodicallydistributed.
 154. An optical fibre according to any one of claims106-153 wherein the inner cladding region acts as a reservoir for lightgenerated in the core.
 155. An optical fibre according to any one ofclaims 106-154 wherein the optical fibre comprises at least two cores.156. An optical fibre according to any one of claims 106-155 whereinsaid at least two cores are adapted for high power broadbandamplification.
 157. An optical fibre according to any one of claims106-156 wherein the fibre core material is the same as the backgroundmaterial of the inner cladding region.
 158. An optical fibre accordingto any one of claims 106-157 wherein said optical fibre is opticallypumped bi-directionally.
 159. An optical fibre according to any one ofclaims 106-158 wherein for a majority or all of said first outercladding features, the minimum distance between two nearest neighbouringouter cladding features is smaller than 1.0 μm.
 160. An optical fibreaccording to any one of claims 106-159 wherein for a majority or all ofsaid first outer cladding features, the minimum distance between twonearest neighbouring outer cladding features is smaller than 0.5 μm,such as smaller than 0.4 μm, such as smaller than 0.3 μm, or such assmaller than 0.2 μm.
 161. An optical fibre according to any one ofclaims 106-160 wherein for a majority or all of said first outercladding features, the minimum distance between two nearest neighbouringouter cladding features is smaller than a predetermined wavelength. 162.An optical fibre according to any one of claims 106-161 wherein for amajority or all of said first outer cladding features, the minimumdistance between two nearest neighbouring outer cladding features issmaller than a prdetermined wavelength.
 163. An optical fibre accordingto any one of claims 106-162 wherein the second outer cladding region ismade of a homogeneous material.
 164. An optical fibre according to anyone of claims 106-163 wherein the largest cross-sectional dimension ofthe first outer cladding features is equal to or below 10 μm.
 165. Anoptical fibre according to any one of claims 106-164 wherein the largestcross-sectional dimension of the first outer cladding features is equalto or below 3 μm.
 166. An optical fibre according to any one of claims106-165 wherein the inner cladding comprises a background material and aplurality of features, said features having a refractive index beinghigher and/or lower than the refractive index of the backgroundmaterial.
 167. An optical fibre according to any one of claims 106-166wherein the inner cladding features have a lower index than therefractive index of the background material.
 168. An optical fibreaccording to any one of claims 106-167 wherein the inner claddingfeatures are voids.
 169. An optical fibre according to any one of claims106-168 wherein the inner cladding features are filled with vacuum, air,a gas, a liquid or a polymer or a combination thereof.
 170. An opticalfibre according to any one of claims 106-169 wherein the inner claddingfeatures have a lower index than the refractive index of the backgroundmaterial, and the inner cladding features have a cross-sectionaldiameter or dimension in the range of 0.3-0.6 times, preferably 0.3-0.5times, more preferably 0.2-0.3times, and most perferably 0.22-0.28 timesa centre-to-centre spacing between neighbouring inner cladding features.171. An optical fibre according to any one of claims 106-170 wherein thecentre-to-centre spacing between neighbouring inner cladding features isan average centre-to-centre spacing.
 172. An optical fibre according toany one of claims 106-171 wherein the centre-to-centre spacing is largerthan 5 μm, in the range of 8-12 μm, or about 10 μm.
 173. An opticalfibre according to any one of claims 106-172 wherein the core comprisesone or more dopants for raising or lowering the refractive index abovethe refractive index of the background material of the inner cladding.174. An optical fibre according to any one of claims 106-173 wherein theouter diameter of the inner cladding is within the range from 60-400 μm,or within the range from 200-400 μm.
 175. An optical fibre according toany one of claims 106-174 wherein the optical fibre has a length with afirst end and a second end, and wherein the cross-sectional area of thefirst outer cladding features in the first end are larger than anycross-sectional area of first outer cladding features in the second end.176. An optical fibre according to any one of claims 106-175 wherein thesecond end comprises no first outer cladding features, or wherein thefirst outer cladding features are fully collapsed in the second end.177. An optical fibre according to any one of claims 106-176 wherein thefirst outer cladding features are elongate features extending in a fibreaxial direction.
 178. An optical fibre according to any one of claims106-179 wherein the inner cladding features are elongate featuresextending in a fibre axial direction.
 179. An optical fibre according toany one of claims 106-178 wherein the background material surroundingthe first outer cladding features or the bridging material fulfillingthe area between neighbouring first outer cladding features has a lowerrefractive index than the refractive index of the background material ofthe inner cladding.
 180. An optical fibre according to any one of claims106-179 wherein the background material surrounding the first outercladding features or the bridging material fulfilling the area betweenneighbouring first outer cladding features has a lower refractive indexbeing less than 4% lower than the refractive index of the backgroundmaterial of the inner cladding, such as less than 3% lower, such as 2%lower, such as 1% lower.
 181. An optical fibre according to any one ofclaims 106-180 wherein the core has a refractive index being less than1.0% different from the refractive index of the background material ofthe inner cladding, such as less than 0.5% different, such as less than0.2% different, such as less than 0.1% different, such as less than0.05% different.
 182. An optical fibre according to any one of claims106-181 wherein the core has a refractive index being equal to or higherthan the refractive index of the background material of the innercladding.
 183. An optical fibre according to any one of claims 106-182wherein the core has a refractive index being equal to or lower than therefractive index of the background material of the inner cladding. 184.An optical fibre according to any one of claims 106-183 wherein theoptical fibre is an endoscope.
 185. A method of producing an opticalfibre for guiding light of at least one predetermined wavelength, themethod comprising: (a) providing a preform, said preform comprising:(vi) at least one centre preform element (144) for providing a coreregion (52) of the optical fibre, said center preform element comprisingat least one element selected from the group consisting of rods, tubes,or combinations thereof; (vii) a plurality of inner cladding preformelements (142, 143) for providing an inner cladding region (53) of theoptical fibre, said inner cladding preform elements comprising at leastone element selected from the group consisting of rods, tubes, orcombinations thereof; (viii) a plurality of first outer cladding preformelements (141) for providing a first outer cladding region of theoptical fibre, said first outer cladding preform elements comprising aplurality of elements selected from the group consisting of rods, tubes,or combinations thereof; (ix) optionally a plurality of further outercladding preform elements for providing at least one further outercladding region of the optical fibre, said further outer claddingpreform elements comprising a plurality of elements selected from thegroup consisting of rods, tubes, or combinations thereof; and (x) anovercladding preform element (140) for providing an outer diameter ofthe optical fibre, said overcladding preform element comprising anelement in form of a tube; and (b) drawing said preform into a fibre;wherein said first outer cladding preform elements are arranged toprovide a minimum distance between two neighbouring first outer claddingelements of the optical fibre which is smaller than the wavelength ofsaid light of at least one predetermined wavelength to be transmitted inthe optical fibre.
 186. The method according to claim 185 wherein saidfirst outer cladding preform elements are arranged to provide a minimumdistance between two neighbouring first outer cladding elements of theoptical fibre which is smaller than 1.0 μm.
 187. A method according toclaim 185 or 186 wherein said at least one centre preform element isdoped with a rare earth element.
 188. A method according to any one ofclaims 185-187 wherein said plurality of inner cladding preform elementsconsists of undoped silica.
 189. A method according to any one of claims185-188 wherein said plurality of inner cladding preform elementsconsists of capillary tubes containing air.
 190. A method according toany one of claims 185-189 wherein said first outer cladding preformelements consist of capilllary tubes containing air, said capillarytubes providing a larger air-filling fraction than that provided bycapillary tubes of said inner cladding preform elements.
 191. A methodaccording to any one of claims 185-190 wherein said tubes and rods havecircular, squared, or rectangular cross-sectional shapes.
 192. A methodaccording to any one of claims 185-191 wherein said drawn fibrecomprises different outer shapes along its length, preferably said drawnfibre (160) has a circular shape at one end (162), and has a rectangularshape at the other end (161).
 193. Use of an optical fibre as defined inclaim 1-105, or as produced by a method as defined in claims 185-192, inan optical laser, optical amplifier, or endoscope.
 194. Use according toclaim 193 wherein said optical laser is a high-power lasers, preferablya cladding pumped laser.